PURPOSE: To detect stably out-of-focus by connecting a frame supporting a collimate optical system and a connecting member connecting the said frame and a light source at a position to make hardly an optical axis shifted against external temperature changes.

CONSTITUTION: A semiconductor laser 30 is supported by the 1st frame 74 and collimator lenses 32, 34 are supported to the 2nd frame 76 respectively. The 1st and 2nd frames 74, 76 are fixed at the same time by a connecting member 72. The 2nd frame 76 and the connecting member 72 are connected by a mounting member 78 at one position in this case and the connected position is a position where a lens face of the collimator lenses 32, 34 closest to the semiconductor laser 30 is moved only within a permissible range to the semiconductor laser 30 due to thermal expansion and contraction, that is, a position in the vicinity of the lens face of the collimator lenses 32, 34 closest to the semiconductor laser 30.