Abstract: A reflecting plate unit includes a reflecting plate (1) attached to an illuminating tool; an LED base plate (10) which is attached to the reflecting plate (1) and to which LED elements are attached side by side in lengthwise direction; a light transmitting cover (20) which includes a substantially cylindrical member with an opening (20a) cut out from one end to the other in lengthwise direction, which is engaged with the reflecting plate (1) in attachable and detachable manner so as to cover the LED base plate (10) and which transmits light in a part or as a whole; engaging portions (30m, 20f) provided in the reflecting plate (1) and the light transmitting cover (20) respectively for causing the reflecting plate (1) and the light transmitting cover (20) to be engaged with each other and allowing them to slide one relative to the other in lengthwise direction.