A combustion chamber structure in which an installation hole of the injector is improved to realize a rapid combustion and to reduce an unburned hydrocarbon. In the combustion chamber, a leading end of an injector is withdrawn from an upper end of an opening of an installation hole, and an additionally expanded surface area is formed on an upper inner surface of the installation hole from a portion to which the leading end of the injector is situated to an opening end of the installation hole. An angle between the additionally expanded surface area and a joint surface of a cylinder head is narrower than an angle between a center axis of the installation hole and the joint surface.