Warm(- 10E4 K), low-density(- 0.1 cm-3) ionized gas is widespread
 over the whole sky with a typical Galactic scale height of 1 Kpc,
 producing the optical H_alpha background (Reynolds 1992). The source
 of ionization is not yet clear, and furthermore, the relationship
 between the warm ionized medium and other interstellar medium such as
 HI clouds and the hot ionized coronal gas, is not understood yet.
 As for the source of the ionization, probably photoionization by
 UV photons may not explain fully  the properties of the warm ionized
 medium observed at optical, and other mechanism, such as ionization
 by low velocity shocks, should be considered. In order to clarify the
 ionization mechanism and to study the origin of the diffuse warm
 ionized medium, we propose far-infrared spectroscopic observations
 using the LWS toward several targets selected from the H_alpha map
 by Reynolds(1992). In the far-infrared, there are also many spectral
 lines as in optical which can be used as diagnostic tools of physical
 condition of the matter. Moreover, observation from space does not
 suffer from contamination by terrestrial emission, while ground-based
 optical observation sometimes suffers. Part of the observation will
 be executed in the Japanese guaranteed time, and this proposed
 observation greatly supplements the guaranteed time observations.