To reduce the power consumption of a portable telephone set adding device being equipment to be used by being connected to the external communication terminal of a portable telephone set.

An adding device 1 is provided with a control means 6 having an external interrupting terminal 9 and a clock 4 for synchronization and a DTMF tone converting circuit 5 for converting a DTMF tone into a serial signal. In this case, the output signal of the DTMF tone converting circuit 5 is inputted to the external interrupting terminal 9, and at the time of receiving the output signal of the DTMF tone converting circuit 5 by the external interrupting terminal 9 when the clock 4 stops, the control means 6 performs the transition of the clock 4 from the stopping state to the operating state, and at the time of not receiving the output signal of the DTMF tone converting circuit 5 in a prescribed time or more when the clock 4 is operating, the control means 6 stops the clock 4.