A battery holder includes storage chamber for storing a battery. The storage chamber includes: opening end surface through which battery can be taken in or out; and upper surface section, lower surface section, and first and second side-surface sections (opposing each other) for holding the battery. Projection is formed on the inner wall of first side-surface section at a portion including the end on the opening end surface side. Outward deformable flexible section is formed on second side-surface section at a portion including the end on the opening end surface side. Slit that extends from third side-surface section opposite to the opening end surface toward the opening end surface and stops at the position corresponding to a part of the battery is formed in upper surface section and/or lower surface section. The end of the flexible section is located closer to the third side-surface section than the projection.