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 (R2) smaller than the outer diameter (R1) 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 (R6) of the coned disc spring at the time of completion of tightening is set to a value greater than the outer diameter (R4) of the rotation operating part and smaller than an outer diameter (R3) 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 (S2) between the coned disc spring and the plain washer is greater than a contact surface area (S1) between a disc shaped collar , provided in a lower portion of the head , and the coned disc spring , and a contact surface area (S3) between the plain washer and a bearing surface on an upper face of the cylinder head is greater than the contact surface area (S2). 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 (R2) smaller than the outer diameter (R1) 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 (R6) of the coned disc spring at the time of completion of tightening is set to a value greater than the outer diameter (R4) of the rotation operating part and smaller than an outer diameter (R3) 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 (S2) between the coned disc spring and the plain washer is greater than a contact surface area (S1) between a disc shaped collar , provided in a lower portion of the head , and the coned disc spring , and a contact surface area (S3) between the plain washer and a bearing surface on an upper face of the cylinder head is greater than the contact surface area (S2). <EOS>