PURPOSE: To simplify fabricating and assembling work and to reduce production cost by providing prisms which guide the beam from a light emitting element to a light receiving element and directly fixing these elements on a printed circuit board.

CONSTITUTION: A pulse light 13 from an LED 25 is refracted by a center prism 17 fixed on a chassis 16 and transmitted to the left and to the right with respect to a reference line at an angle of 7 degrees. The divided pulse light 13 pass transparent leader tape portions 21a and 21b of the both sides of a video tape 21, enters into left and right side prisms 18 and 19 through external side light inspection holes 15c and 15d and received by phototransistors 26a and 26b which are light receiving elements installed on a printed circuit board 23. In that case, the LED 25 is placed so as to face the phototransistors 26a and 26b across e runway of the video tape 21 and the center prism 17 is set at an angle where the light receiving efficiency through the holes 15a to 15d becomes maximum.