  Most of the observational studies of supernova (SN) explosions are limited to
early phases (< a few yr after the explosion) of extragalactic SNe and
observations of SN remnants (> 100 yr) in our Galaxy or very nearby galaxies.
SNe at the epoch between these two, which we call "transitional" phase, have
not been explored in detail except for several extragalactic SNe including SN
1987A in the Large Magellanic Cloud. We present theoretical predictions for the
infrared (IR) dust emissions by several mechanisms; emission from dust formed
in the SN ejecta, light echo by circumstellar and interstellar dust, and
emission from shocked circumstellar dust. We search for IR emission from 6
core-collapse SNe at the transitional phase in the nearby galaxies NGC 1313,
NGC 6946, and M101 by using the data taken with the AKARI satellite and
Spitzer. Among 6 targets, we detect the emission from SN 1978K in NGC 1313. SN
1978K is associated with 1.3 x 10^{-3} Msun of silicate dust. We show that,
among several mechanisms, the shocked circumstellar dust is the most probable
emission source to explain the IR emission observed for CSM-rich SN 1978K. IR
emission from the other 5 objects is not detected. Our current observations are
sensitive to IR luminosity of > 10^{38} erg s^{-1}, and the non-detection of SN
1962M excludes the existence of the shocked circumstellar dust for a high gas
mass-loss rate of sim 10^{-4} Msun yr^{-1}. Observations of SNe at the
transitional phase with future IR satellites will fill the gap of IR
observations of SNe with the age of 10-100 years, and give a new opportunity to
study the circumstellar and interstellar environments of the progenitor, and
possibly dust formation in SNe.

Solar and Stellar Astrophysics