PURPOSE: To reduce consuming power automatically for an increase in the dischargeable time of a battery for interior roof lamp when disconnecting a connector for use as a flash lamp by improving a portable interior roof lamp unit which can be attached and removed to/from an on-vehicle battery through the connector with an electric discharge lamp serving as a light source and can be used as the flash lamp.

CONSTITUTION: An NPN transistor 8 controls the output of an inverter circuit. The base current Ib3 of the transistor is supplied through operating current control resistors 9a and 9b connected in parallel. When a connector 2 is disconnected, the current passing through the operating current control resistor 9b becomes zero, and the output of the inverter circuit is reduced. It is thus possible to reduce and save the discharge current of a battery 4 for interior roof lamp although the luminous energy of an electric discharge lamp 12 decreases.