The 155 mu m continuum emission from the galactic plane around l = 50(deg) was observed with an 8'times13 ' beam by the Far-Infrared Line Mapper (FILM) aboard the Infrared Telescope in Space (IRTS). The observed map noticeably resembles the IRAS 100 mu m map. The dust temperature calculated from their flux ratio is almost constant (16--18 K) throughout the observed area, except for the HII regions. The dust temperatures in the HII regions are significantly higher than in the surrounding region; there is a clear boundary between the cold and hot regions. The observed uniformity of the dust temperature, except for in the HII regions, implies that the strength of the average interstellar radiation field (ISRF) along each line of sight is almost constant (i.e., varies by less than a factor of 2) in the observed region of the galactic plane.