In a transmission circuit, a first pulse signal with a first frequency and a second pulse signal with a second frequency are output according to a rising edge and a falling edge of a first input signal, respectively. When a second input signal indicates an active level, the second pulse signal is output according to the falling edge of the first input signal and the second frequency is changed to a third frequency. In a reception circuit, a first level of a first output signal is changed to a second level according to a first induced signal via a transformer, the second level of the first output signal is changed to the first level according to a second induced signal via the transformer, and a second output signal is changed to an active level when a frequency of the second induced signal has changed to the third frequency.