INTERNAL COMBUSTION ENGINE

PURPOSE:To stabilize output regardless of the compression ratio in an internal combustion engine forming a combustion chamber loaded with a sparking plug in its cylinder head by constituting a part of the combustion chamber including the sparking plug by a different block capable of being advanced or retracted in a cylinder head body. CONSTITUTION:In a cylinder head 2, a combustion chamber 7 is formed in accordance with a cylinder 1c. The middle part of the combustion chamber 7 is constituted by a block 2b different from a cylinder head body 2a, and a sparking plug 8 is loaded into the center part of the block 2b. On the block 2b, thread parts 9a, 9b in the two, upper and lower steps are provided, and the block is screwed into the cylinder head body 2a with the thread part 9a so that the block can be advanced or retracted, and further the thread part 9b fixes the block 2b positively. When the compression ratio is changed, the block 2b is advanced or retracted with respect to the cylinder head body 2a with the sparking plug 8 held integrally with it, and thereby the relative position of ignition part of the sparking plug 8 and the surrounding wall surface of the combustion chamber is not changed in any way.