An electronic control system includes a microcomputer and a signal processing circuit, and receives output signals of an in-cylinder pressure sensor and a crank angle sensor. The output signal of the pressure sensor indicates an output voltage of a piezoelectric transducer and a reference voltage. The signal processing circuit includes a hold circuit and a differential amplifier circuit. The differential amplifier circuit amplifies a difference between the output signal of the pressure sensor and the output signal of the hold circuit and outputs the result to the microcomputer. The hold circuit switches over the output signals in response to a switchover instruction of the microcomputer. When the switchover instruction is a set instruction, the hold circuit holds as its output signal the output signal of the pressure sensor outputted at the time of receiving the set instruction. The microcomputer outputs the set instruction to the hold circuit in response to the output signal of the crank angle sensor.