PURPOSE: To provide a diesel engine having a compression ratio changeable means with which a compression ratio can be changed to a low compression ratio under a high temperature, or to a high compression ratio under a low temperature automatically.

CONSTITUTION: A combustion chamber block 5 of ceramic of a low heat expansion ratio is disposed at a cylinder head 3, and a ring-like auxiliary chamber 2 is formed in an outer circumference of a liner upper part 12 of the combustion chamber block 5. Plural communication holes to let the ring-like auxiliary chamber 2 with a main chamber 1 are formed in the combustion chamber block 5. A ring 6 manufactured of metal is disposed inside the ring-like auxiliary chamber 2, and a protruding part 7 extended inward along the radius is composed in an integrated structure with the ring 6. The protruding part 7 is protruded into the communication hole 8 to close the communication hole 8 under a low temperature at the time of starting or the like for achieving a high compression ratio. In the meanwhile, the ring 6 is thermally expanded when the combustion chamber is at a high temperature to release the communication hole 8 for achieving a low compression ratio.