To provide a lighting circuit which can make full lighting of a lamp of a light source for an underwater light or the like.

The lighting circuit is provided with a lamp 23 and a dimming circuit including a transistor TR to convey a load current for dimming to the lamp 23 both connected in parallel against a driving battery 22, and the load current is controlled by the dimming circuit and the lamp 23 is lit while a dimming is conducted. There are provided a connection point between the lamp 23 and the transistor of the dimming circuit, a relay contact point RYC connected with the ground and a relay RY connected with a battery 22 with a switch S in-between, and a full lighting controlling circuit is provided in which the relay RY is excited by switching on to switch on the relay contact RYC and the transistor TR is short-circuited and all of the driving voltage from the battery 22 is applied to the lamp 23.