PURPOSE: To make it possible to detect the breakage of a lamp without breaking a lamp circuit by constructing a switching circuit so that the secondary coil of a transformer may be connected in parallel with a lamp when the breakage of the lamp is tested.

CONSTITUTION: When a projecting main switch S4 is broken, a lamp breakage detecting relay Ry is restored so that the common contact of a relay contact of a relay contact Rc is connected with a lamp side. By impressing the pulse voltage of Vb1 to the base of a switching transistor, the composed current of currents iL and iT flows through a resistor R3 thereby to generate a voltage V1. As a result, a transistor Tr2 is rendered inconductive to leave a breakage indicating lamp LED unlit. However, if a lamp L is broken, the current iL is reduced to zero so that the voltage V1 comes close to zero. As a result, the transistor Tr2 is rendered conductive so that the lamp LED is lit.