A cap for installing a semiconductor device that can send or receive a light having a predetermined wavelength, the cap including a recess for installing the semiconductor device, the recess being defined by a through-hole penetrating an upper surface of a silicon substrate and a lower surface of the silicon substrate, the through-hole having an upper end part of the through-hole on a side of the upper surface of the silicon substrate and a lower end part of the through-hole on a side of the lower surface of the silicon substrate, and a coating film formed to cover the upper surface of the silicon substrate and the upper end part of the through-hole, wherein the coating film that covers the upper end part of the through-hole is a window part that transmits the light having a predetermined wavelength.