An imaging element mounting substrate includes: an insulating base comprising insulating layers, the insulating base surface comprising an opening which is located at a center thereof; a connection electrode disposed at a lower surface of the insulating base around the opening; and light-transmission control layers between the insulating layers, and comprising inner edges located on an outside of the opening. An inner edge of one light-transmission control layer lies closer to the opening than an inner edge of another light-transmission control layer as seen in a transparent plan view, and the insulating base has, in an inner periphery of the opening of an insulating layer constituting a lower surface of the insulating base, an inclined portion which is inclined such that the opening becomes smaller in size from the lower surface as approaching an upper surface of the insulating base.