To provide a hook lock monitoring device simply and easily fitted to a hook lock externally, which is provided in a sliding door and a hinged door, to detect the opening/closing.

A hook lock 10 fitted to a sliding door 34 performs locking by fitting a hook piece 18 into a latch port of a metal latch fitting by locking operation and performs unlocking by withdrawing the hook piece 18 from the metal latch fitting by unlocking operation. A microswitch 42 is provided in a hook lock monitoring device 40, a detection lever 46 for operating a switch knob 44 is inserted into a clearance between a mating surface 34a of the sliding door 34 and the corresponding mating surface, motion of the hook piece 18 according to the locking operation or unlocking operation of the hook lock 10 is detected by the detection lever 46 to turn on/off the microswitch 42, and a detection signal showing locking or unlocking of the hook lock is output.