A lid mechanism includes: a box main body; a lid; a locking element that includes a rod and a rod biasing member; and a flap element that includes a flap main body and a flap biasing member and a box contact portion which is configured to come into contact with the box main body, wherein: when the state of the lid changes from the closed state to the opened state, the pressing inclined surface provided in the rod contact portion of the flap main body presses the pressure receiving inclined surface provided in the rod; and when the state of the lid changes from the opened state to the closed state, the box contact portion of the flap main body comes into contact with the box main body such that the pressing of the pressing inclined surface to the pressure receiving inclined surface is released.