To provide a three-line system sensor output processing circuit with a self-diagnosis function which enables precision increase by an increase of a dynamic range of voltage of a transmission signal, without increasing transmission lines.

A sensor 1 is judged to malfunction, in the state of the dynamic range of the signal voltage Vs being limited to a range in which it is lower than the higher-side source potential Vcc of a power source by a voltage margin ΔV, or higher than the lower-side source potential 0 V of the power source by the specified voltage margin ΔV. At that time, the malfunctioning state is transmitted with the voltage Vo of an output signal kept at a potential level higher furthermore than a maximum potential of this signal voltage Vs, or with the potential of a signal output terminal 7 kept at a potential level lower than a minimum potential of this signal voltage Vs. Consequently, precision of the three-line system sensor output processing circuit with a self-diagnosis function can be increased, since malfunctions can be reported without reducing the dynamic range of the signal voltage.