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SCIENTIFIC ABSTRACT There are still a number of problems in the nature of the diffuse interstellar medium (DISM), which have to be investigated by observations. The questions to be addressed involve a clear identification of various phases of the DISM, their volume or mass fractions, their roles and interrelations, the transition processes from one to another phase, and the properties and size distribution of dust grains in the DISM. The gas and the dust both play important roles in the energy and mass flow in the DISM. The strength of the interstellar radiation field controls the energy input, giving a crucial influence on the physical and chemical conditions of the DISM. It is therefore very valuable to observe both the gas and the dust in the same medium under various radiation field conditions. The emission spectrum from the DISM peaks at the far-infrared region and the gas and dust behaviors are best studied by the far-infrared spectroscopy. The near-to-mid infrared spectroscopy provides with the supplemental information on the gas species that have transitions in this wavelength region and on the dust grains of very small size population. LWS, SWS, and PHT-S grating spectral mapping observations of selected regions of various physical conditions, in particular, those under various radiation field strengths elucidate the physical transition processes of the phases and the effects of the interstellar radiation field. Supplemental information on the dust grains is provided by the near-to-mid IR extinction observation of the DISM in the Galactic plane by CAM CVF. Observations of external galaxies of different metallicity by the LWS line spectral scan reveal the roles of dust grains in the DISM from a different viewing point. In addition, FIR polarization observations are proposed separately to study the dust components contributing to the FIR continuum emission (II. Polarization Study). Its targets are coordinated to this proposal. OBSERVATION SUMMARY The observation of this proposal is roughly divided into three parts: (1) Two-dimensional mapping of the typical active diffuse interstellar medium. This will provide with the identification of various phases consisting of the active DISM and with their roles in a typical DISM. (2) One-dimensional scan of the warm regions where the strength of the radiation field is considered to change with the scanning direction and multi-point observation of the regions under the normal interstellar radiation field. This set of observation will provide with the properties of the DISM under various radiation field strengths. (3) Supplemental observations including the line spectrum observation of dwarf galaxies of different metallicities and the extinction observation of star clusters in the Galactic Center. For observation of (1), Carina region is selected as the target. The rectangular area enclosed by 287 < l < 287.65 and -47' < b < -26' together with the long strip of 3' width between -83' < b < +10'(l = 287.325) are mapped with 3' grid in the raster scan mode. In addition to these areas, 6 points away from the edges of the strip in the both sides are observed to define a base line. Full SWS low-resolution grating spectra are obtained for 4 selected positions in order to investigate the line emission and dust continuum in the 12 -45 micron region. Two well-studied HII regions, W1 and North America and Pelican Nebula (NPN), are selected as targets of category (2). One dimensional raster scan of 24 points with 50" grid is carried out for W1, while that of 32 points with 180" grid is attempted for NPN. A similar one-dimensional scan of 3' or 6' grid is performed for reflection nebulae (see next section for target name). The Ursa Major cirrus cloud and the IR excess cloud (Desert et al. 1988) are chosen as the targets under the normal interstellar radiation field. Three or four positions are observed for each cloud. The positions to be observed are carefully selected from the IRAS data. The warm ionized medium (WIM) revealed by H alpha observation (Reynolds 1992) is also selected as the object of this category. The observed positions are determined from the IRAS data and the H alpha data. Dwarf galaxies with known metallicities are selected as targets in category (3) observation (for target name, see next section). For faint galaxies, observations of four reference positions (cross) are also made in the raster scanning mode. The extinction observation is made for three star clusters in the Galactic Center region.