Propagation time measurement device and electronic key system

An electronic key system in which a transmission signal is transmitted, through wireless communication, from a vehicle to an electronic key and returned to the vehicle from the electronic key, wherein a plurality of continuous waves having different frequencies are combined to generate the transmission signal, the electronic key system comprising

• a propagation time measurement device that measures a propagation time of the transmission signal from when the vehicle transmits the transmission signal to the electronic key to when the transmission signal returns to the vehicle from the electronic key, wherein the propagation time measurement device includes
• a correlation computation unit that generates a plurality of correlation values by performing a correlation computation on each of the continuous waves and a reception signal that is returned from the electronic key and received by the vehicle, and
• a propagation time measurement unit that measures the propagation time from a phase of the correlation value generated for each continuous wave in the correlation computation.