A protrusion is formed on a facing surface which is integral with a shaft of a hinge mechanism. The protrusion has a tip that is in contact with an end surface of the engagement portion when the first member and the second member swing relative to each other, so that displacement of the second member along the lengthwise direction of the shaft is restricted. A state immediately after the engagement portion is engaged with the shaft is referred to as an initial engagement state. In the initial engagement state, the protrusion is situated at a position beyond a center of the shaft when viewed in a direction of engaging the engagement portion with the shaft, and the end surface of the engagement portion is separated from the tip of the protrusion.