PURPOSE: To prevent the malfunction due to external noise and to improve the reliability by detecting a data signal having the start and set signals of dissimilar codes before and after a set data length.

CONSTITUTION: At the transmission and reception sides, the code of the start signal, the code of the set signal, data signal, and data length are set to correspond each other. When the signal sent from a transmitter is received, the presence or absence of the start and set signals constructing each one signal by a DTMF receiver 9 is detected by a start/set code comparison section 11 comparing the set start/set signals. Also, the data length of the data signal can be detected by a reception counter comparison section 13 comparing the data length with the set data length. When the detected value is normal, it is outputted as a normal signal through an output gate 15, and when it is abnormal, outputted as a reception error. Thus, the receiver side can easily confirm the normal and abnormal signals.