PURPOSE: To eliminate troubles of turning or and turning off an illuminating lamp switch each time when picture projection is interrupted, by controlling the indoor illumination in accordance with a lighting switch for picture projection.

CONSTITUTION: Two fixed pieces 18 and 19 and a mobile piece 21 which is normally brought into contact with them are arranged in an interlocking input part 5. Two fixed pieces 18 and 19 are connected to a part placed in the power source side of a power line 1A for illumination and a part placed in the illuminating lamp side respectively. Consequently, when an insertion part 17 is inserted to the interlocking input part 5, the mobile piece 21 is moved by a projection provided on the insertion part 17 and is separated from fixed pieces 18 and 19. Simultaneously, end parts of two connection cords 13 and 14 are brought into contact with corresponding fixed pieces 18 and 19. Thus, an illuminating lamp 6 is lit if a lighting switch 9 is turned off when an illuminating lamp switch 4 is turned on. If the lighting switch 9 is turned on when the illuminating lamp switch 4 is turned on, the illuminating lamp 6 is extinguished.