A lock detecting device (10) which is capable of detecting a state of a thumbturn (1) and transmitting a detection result and is very versatile and device costs of which are small is provided. A sensor portion that detects whether the state of the thumbturn (1) is in a first state or in a second state orthogonal to the first state and a wireless communication portion that transmits a signal corresponding to a detection result of the sensor portion by wireless communication are included.