A semaphore control system switches a signal lamp with usual filament and standby filament to use the standby filament by blowing a repair contact of a detection relay when the usual filament is blowed. The present invention relates to blowing detection of usual filaments, wherein the blowing of the usual filament is detected by a simple structure that a signal representing the blowing of the usual filament need not be inputted to a control cable or an input circuit of a semaphore control device from the signal lamp, such that economical performance may be improved. An output relay (FDR) of afilament breakage detector (10) for detecting the blowing of the usual filament (5) has a slow-release function of a repair time (Td), when a semaphore control part (2) detects that a periodic signalof power supply is normal, if a lamp current detection signal inputted from a lamp current detection part (4) is switched off under a condition of switching on the control output of the lamp, the switching-off time is calculated, when the duration time of switching off the lamp current detection signal is more than a power supply switching time (Ts) and is identical with the repair time (Td) of the output relay (FDR) of the filament breakage detector detector (10), the blowing of the usual filament (5) is detected.