In an ON state, a first switch generates a transmission signal by pulse modulating a carrier. A SAW sensor outputs a reception signal after a delay from receipt of the transmission signal. In an input state, a second switch receives and transmits the transmission signal to the SAW sensor. In an output state, the second switch receives and outputs the reception signal. A SAW delay element outputs a local signal after the delay from receipt of the transmission signal. A mixer generates a synchronous detection signal by mixing the reception and local signals. During the input state of the second switch, the first switch switches from the OFF state, to the ON state, and back to the OFF state. The second switch switches from the input state to the output state before the delay after the first switch switches back to the OFF state.