PURPOSE: To decrease overshooting quantity at the tube wall temperature and stabilize optical output of the captioned device, by disposing around a discharge lamp two groups of heater elements which are connected in parallel and by on- off controlling each group in each different temperature range.

CONSTITUTION: When heating a fluorescent lamp used for an exposure lamp or the like of a copying machine, 1st and 2nd heater elements 6, 7 are buried in parallel in an insulating material installed around said lamp and these elements 6, 7 are on-off controlled in individually different temperature ranges by means of 1st and 2nd thermostats 8, 9 mounted on the lamp tube wall. Consequently, in a low temperature environment and soon after a power source is actuated, the lamp is rapidly heated by the two heater elements 6, 7, the temperature of the tube wall is raised up to a temperature which is a little lower than optimum temperature of 40°C, one of said heater elements is turned off, and the other is forced by control the temperature and by doing that the overshooting quantity can be decreased and a stable optical output can be obtained.