A thermally sprayed coating is formed on an inner surface of a cylinder bore of a cylinder block by using a thermal spray gun. The thermal spray gun is reciprocated along an axial direction in the cylinder bore while being rotated, and injects melted droplets generated by melting a wire made of a ferrous material from a nozzle at its end. At this time, a moving speed of the thermal spray gun along the axial direction into the cylinder bore is made equal-to or larger-than a predetermined value, and the number of reciprocating cycles of the thermal spray gun along the axial direction into the cylinder bore is made equal-to or larger-than a predetermined value.