A heat receiver, a reactor, and a heater utilize the heat of concentrated solar light for thermal decomposition and/or chemical reaction of coals, etc. The heat receiver includes: a side portion forming a substantially cylindrical side surface; a substantially circular bottom portion connected to the lower edge of the side portion; and a ceiling connected to the upper edge of the side portion. A substantially circular aperture is formed in the center of the ceiling. The heat receiver has a substantially cylindrical cavity and the opening portion is open. When the cavity has a diameter of D and a length of L, and the aperture has a diameter of d, d=D/2 or less and L=2D or more. Concentrated solar light entering the heat receiver is to be contained in the heat receiver to effectively utilize the solar light.