A capacitor charged beforehand is discharged according to a light surrounding a display unit. A data is decreased similarly to the voltage between the electrodes of the capacitor. A trigger signal is outputted if the data becomes equal to or less than a threshold value. A clock signal whose cycle of changing levels gradually becomes long is generated. A count value is updated at each change of the clock signal's level and the updated count value is outputted. The count value is sampled when the trigger signal is outputted.