To provide a seal structure of an engine, which does not increase the number of man-hours for forming the thick portions at the end part of the cylinder head side of a sleeve or the like, for example.

According to the seal structure of an engine, an aluminum alloy sleeve 27 is cast in a cylinder bore wall 22d of an aluminum alloy cylinder body. A gasket 91 is interposed between the cylinder block, in which a plate coating a is formed on the inner surface of the sleeve 27, and cylinder head 23, and the two are fastened by a head bolt. The aluminum alloy of the cylinder body contains 5 to 20 wt.% of Si. The aluminum alloy of the sleeve 27 contains Si that satisfies the equation 50<α<150 (in the equation, a represents 100 times the Si weight percentage content of the sleeve 27 divided by the Si weight percentage content of the cylinder body). In addition, the seal point P of the gasket 91 is abutted against both surfaces 22f, 27g of the cylinder body and the cylinder head 23 of the sleeve 27.