PURPOSE: To adjust the position of a light reception element easily with a simple tool.

CONSTITUTION: The title device is provided with an enclosure 1 where the incidence direction of light is opened and an adjustment hole 1d which penetrates in square is provided at the side, a light reception element 2 which converts the received incidence light to an electrical signal, and a light reception element holder 3 which retains the light reception element 2 and at the same time a recessed part 3d where the width in a direction crossing the incidence direction of light is smaller than the adjustment hole 1d is provided at a position corresponding to the adjustment hole 1d and is mounted to the enclosure 1 so that it can slide in the same direction. Then, when the position of the light reception element 2 is adjusted, a tip part 4a is engaged to the recessed part 3d by inserting an adjustment rod 4 with a square section, for example, a near-by minus screwdriver from the adjustment hole 1d for translating in the direction crossing the incidence direction of light for adjustment, and at the same time the adjustment hole 1d is increased as compared with the sectional shape of the adjustment rod 4, thus enabling the adjustment hole 1d to be a mere insertion port of the adjustment rod 4 and the adjusting rod 4 to be inserted into the adjustment hole 1d easily.