An illumination device system includes: an illumination device including an engaging portion configured to engage with an illumination device support base, a main body portion, a light emitting portion configured to swing around a turning axis in relation to the main body portion, and a cable extending from the main body portion; and a diffuser, and the diffuser swings around the turning axis between an advanced position positioned in front of the light emitting portion and a withdrawn position withdrawn from the front of the light emitting portion, and includes a first locking portion configured to engage with the illumination device support base and holds the diffuser at the advanced position, and a second locking portion configured to engage with the cable and holds the diffuser at the withdrawn position.