To obtain a speech path interface protective device preventing excessive current consumption of a reception amplifier.

A 'all '0' PATTERN DET' 6 detects all '6' data forming a negative polarity maximum amplitude by an octet unit after a reception digital input DRin is decoded into analog data. When the pattern of this all '0' data is continuously detected, a 'CONT' 7 judges that a fault exists on a transmission side or in the process of a repeating. Receiving this judgment, an 'IDLE CODE INS' 8 inserts the idle data of all '1' forming a minimum amplitude by the octet unit after the idle data is decoded into analog data. Therefore, when an abnormal reception signal is decoded into analog data, the excessive power consumption of a power source for negative polarity amplification (a minus power source) of a reception side Amp can be suppressed.