To provide a method for manufacturing a cylinder block with which detective part, such as casting flash is not developed after casting and a collapsible core is easily taken out.

In the method for manufacturing the cylinder block having; a core manufacturing process for manufacturing a water-jacket forming core 20 with a metallic insert 10 and a collapsible member 15 for forming the water jacket 57; a casting process for casting the cylinder block 50 by setting the water jacket forming core 20 in outer molds 31, 32; and an after-treatment process for removing the collapsible member 15: the core manufacturing process is such a state, as that the metallic insert 10 is composed of a base part 11 and an inserting part 12, and the inserting part 12 is provided so as to intermittently surround a cylinder bore part 54, and the collapsible member 15 is formed in the whole integral periphery of the inserting part 12; and the casting process is performed, with which a bridge part 53 is formed by making flow of molten metal into a space at the basic portion having no-covering with the collapsible member 15 in a discontinuous part of the inserting part 12 in the metallic insert 10.