A lid opening/closing device includes: a base member provided with an opening; a lid configured to open/close the opening; and a positioning portion configured to position at least a vertical position of the lid relative to the base member at a closed position of the lid. The positioning portion includes an engagement recess portion provided to the base member in a recess shape, and an engagement projection portion provided to the lid in a projection shape and configured to be engaged with the engagement recess portion at least in an up-down direction at the closed position of the lid. The engagement recess portion includes a rib extending in the up-down direction at the closed position of the lid. The rib is engaged at a distal end thereof with the engagement projection portion at the closed position of the lid.