To manufacture a semi-closed deck type cylinder block of high quality and durability by integrally connecting upper parts of a water jacket 13 by using a bottom piece and cladding by welding for closing a gap above the bottom piece.

This is a method of manufacturing a semi-closed deck type cylinder block 10 by integrally connecting an inner peripheral wall surface 19 with an outer peripheral wall surface 20 of a water jacket 13 in the periphery of a cylinder bore 12 of an open deck type cylinder block 11 by using a bottom piece 15 and cladding by welding 16, 17 and 18. The open deck type cylinder block 11 includes a pair of bottom piece housing recessed parts 21 and 22. The following processes are performed: a process for fitting the bottom piece 15, for accommodating, in lower parts of opposite surfaces and bottom surfaces of step parts of the bottom piece housing recessed parts 21 and 22; and a welding process for performing cladding by welding 16, 17 and 18 with a MIG welding machine.