The origin of the IR emission of normal galaxies and its relation
 to the properties of galaxies is still poorly understood.  The IR
 emission of galaxies typically consists of several components and
 can be discussed in the framework of an analogue to the diffuse
 IR emission in our Galaxy.  However, the clear identification of
 and the origin of each component still remain open problems.
 Low-resolution spectroscopy in a wide wavelength region over the
 entire galaxy will be the most useful means to study the global
 nature of each component of the IR emission from external
 galaxies.  We propose to investigate the IR properties of galaxies
 that will be observed by the mid-IR spectrometer (MIRS), an
 instrument on the IR survey mission, the IR Telescope in Space
 (IRTS).  The ISO data will complement the MIRS/IRTS data in
 determining the nature and origin of the IR emission from
 galaxies.  The MIRS is a low-spectral-resolution and wide-field-
 -of-view spectrometer suited to observations of the mid-IR
 emission from galaxies.  To complement the MIRS/IRTS data we
 propose here to take LWS01 full grating spectra as well as CAM01
 observations to cover the 5 to 200 micron range and identify the
 components of the IR emission from galaxies unambiguously.  The
 CAM observation will also provide complementary information on the
 spatial distribution of the shorter IR wavelength emission.  The
 ISO observations together with MIRS/IRTS observations will allow
 us to (i) separate and identify each galaxy emission component
 clearly and (ii) to investigate each component in detail.  With
 these results we can investigate the relation of the components
 with each other as well as with the other properties of galaxies
 and better understand the IR emission from galaxies.