PURPOSE: To provide a train detector for controlling alarm in a crossing alarm, which is not operated instably caused by rust on a rail or a weather.

CONSTITUTION: Optical fibers 13, 23 for transmitting laser from a photo-emitter to a photo-receiver is laid between rails in the vicinity of an alarm starting point 1 and at an alarm ending point 5 so that the transmission of the laser through them is cut off by a pressure or a strain applied by an axle of a train on approaching. A dual system fail-safe computer 40 is provided to the photo- emitter and the photo-receiver so that when the transmission of the laser through the fibers 13, 23 is cut off, it is determined that a train has passed but when it is not cut off, it is determined that no train has passed. A relay AR which is restored when the microcomputer 40 determines that a train has passed at the alarm starting point, and a relay BR which is energized when the microcomputer 40 determines that the train has passed the alarm ending point, are provided so as to restore and energize an alarm control relay R.