In one example, a signal processing circuit including a directional coupler and a termination part is disclosed. The directional coupler includes a main line as a transmission path of an RF signal and a sub-line constituting a coupled line together with the main line. The termination part includes devices connectable between ground and a first port at an end of the sub-line. The signal processing circuit switches, depending on a frequency of the RF signal, the devices of the termination part to be connected to the first port. The phase of a return signal of a signal input as a coupling signal to the termination part via the first port is opposite to the phase of an isolation signal supplied to a second port at the other end of the sub-line and connected to an output port of the coupling signal.