PROBLEM TO BE SOLVED: To provide an automatic dimmer for tunnel illumination, capable of properly dimming and controlling illumination inside a tunnel, even if automatic illumination control matching illuminance and brightness outside the tunnel is disabled. SOLUTION: A compensating circuit includes a failure detection circuit 8 for determining whether or not a detection signal from a light receiver circuit 3 is normal, a timer circuit 10 for counting time and for outputting time signals, a detection signal database control circuit 9 for storing either the detection signal at a predetermined time according to the time signal transmitted from the timer circuit or corresponding data as detected signal data and for managing the previous detection signal data stored, and a switch circuit, which when the detection signal is determined to be normal by the failure detection circuit, outputs the detection signal to a control circuit and, which, when the detection signal is determined to be not normal, outputs in place of the detection signal a compensation signal, based on the detection signal data within the previous predetermined period at the corresponding time stored in the detection signal database.
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