To provide an early fire detecting method for a water-cooled semiconductor bulb capable of early detecting an overheat occurring at any portion of the bulb and installed with sampling heads at positions easy for maintenance.

Sampling heads detecting fine particles are installed on a suction port 8 on the side wall of a bulb hole and at positions apart in the horizontal direction from the suction port 8 on the side wall of the bulb hole. Fine particles generated at the lower section of a bulb apart from the suction port 8 and difficult to detect in the past pass near the side wall on the AC side or DC side and reach the side wall having the suction port 8 then reach the suction port 8, thereby the fine particles can be detected by sampling heads 13-1, 13-2 installed at the positions apart in the horizontal direction from the suction port 8 before they are detected by a sampling head 12 on the suction port 8.