In a semiconductor integrated circuit device, a signal processing circuit includes an output control circuit and an output circuit and is composed of a plurality of MOS transistors. The signal processing circuit inputs a circuit signal and processes the circuit signal. A diagnostic circuit includes at least a diagnostic resistor indicative of a gate length of each of the plurality of MOS transistors and generates a diagnosis signal based on a resistance value of the diagnostic resistor. The output control circuit controls the output circuit to one of an output enable state and an output disable state based on the diagnosis signal.