A substrate holding structure includes holding units and protrusions. The holding units are provided to a casing that includes an accommodating unit accommodating therein an inserted substrate, and hold an end part of the substrate accommodated in the accommodating unit on the insertion direction-entrance side of the substrate. The protrusions protrude from side wall surfaces that are wall surfaces of the casing along the insertion direction of the substrate, the wall surfaces being opposed to the side surfaces of the substrate, and are plastically deformed due to contact with the substrate to be inserted into the accommodating unit.