Engine, comprising a tightening structure for a cylinder head bolt to tighten a cylinder head to an engine casing by threadingly engaging a cylinder head bolt, which has been inserted through the cylinder head, into the engine casing including the cylinder and a crankcase, the tightening structure comprising a plain washer, wherein said tightening structure comprises: a spring member placed over the plain washer to exert a spring force in an axial direction of the bolt, the plain washer and the spring member being disposed between a rotation operating part provided in a head of the cylinder head bolt and an upper surface of the cylinder head, wherein the cylinder head bolt comprises a tension bolt in which an intermediate columnar part has an outer diameter smaller than the outer diameter of an upper end portion, provided with the rotation operating part, and a lower end portion provided with a male threaded region, wherein the spring member comprises a coned disc spring, and the outer diameter of the coned disc spring at the time of completion of tightening is set to a value greater than the outer diameter of the rotation operating part and smaller than an outer diameter of the plain washer, characterized in that a first frictional resistance between the rotation operating part of the cylinder head bolt and the spring member is chosen to be lower than a second frictional resistance between the spring member and the plain washer and a third frictional resistance between the plain washer and a bearing surface defined on the upper surface of the cylinder head, and a contact surface area between the coned disc spring and the plain washer is greater than a contact surface area between a disc shaped collar, provided in a lower portion of the head, and the coned disc spring, and a contact surface area between the plain washer and a bearing surface on an upper face of the cylinder head is greater than the contact surface area. a spring member placed over the plain washer to exert a spring force in an axial direction of the bolt, the plain washer and the spring member being disposed between a rotation operating part provided in a head of the cylinder head bolt and an upper surface of the cylinder head, wherein the cylinder head bolt comprises a tension bolt in which an intermediate columnar part has an outer diameter smaller than the outer diameter of an upper end portion, provided with the rotation operating part, and a lower end portion provided with a male threaded region, wherein the spring member comprises a coned disc spring, and the outer diameter of the coned disc spring at the time of completion of tightening is set to a value greater than the outer diameter of the rotation operating part and smaller than an outer diameter of the plain washer, characterized in that a first frictional resistance between the rotation operating part of the cylinder head bolt and the spring member is chosen to be lower than a second frictional resistance between the spring member and the plain washer and a third frictional resistance between the plain washer and a bearing surface defined on the upper surface of the cylinder head, and a contact surface area between the coned disc spring and the plain washer is greater than a contact surface area between a disc shaped collar, provided in a lower portion of the head, and the coned disc spring, and a contact surface area between the plain washer and a bearing surface on an upper face of the cylinder head is greater than the contact surface area.