PURPOSE: To prevent the delay in emission stoppage at the automatic light adjusting during internal light photography and achieve the longer life of thyristors by blocking the supply of the gate current which fires an internal light tube through application of reverse voltage to thyristors.

CONSTITUTION: When the light emission quantity of an internal light discharge tube V1 becomes of a specified quantity, the detection output of a light quantity detecting circuit LED causes a transistor Tr2 to conduct. Then a thyristor SCR3 goes ON and an arrester A1 is excited by the trigger pulse of a pulse transformer T2. This causes a capacitor C6 to discharge to flow reverse current in a thyristor SCR1. This current is stored in the capacitor C5. The discharge current of this capacitor C5 flows in the bypass circuit by turning ON of the transistor Tr1 via capacitor C8 by conduction of the thyristor SCR3, thus the gate current being applied to the thyristor SCR1 by the capacitor C6 is blocked. Hence, thyristor SCR1 is not refired and goes OFF without delay after it has been turned OFF by the reverse current.