PURPOSE: To suppress the deformation of a cylinder bare at the time of grinding by mounting a deformation preventing member for suppressing the radial deformation of the cylinder bore, to an open deck type cylinder block, and in this state, grinding the inner surface of the cylinder bore.

CONSTITUTION: A deformation preventing device 11 for suppressing the deformation of a cylinder bore 7 at the time of grinding (honing) the inner surface of the cylinder bore 7 is set onto the upper face of a cylinder block 5. In the deformation preventing device 11, the center of a device body 13 is provided width a through hole 15 with a diameter slightly larger than the inner diameter of the cylinder bore 7 so as to be adjusted to the cylinder bore 7. Plural auxiliary pins 17 serving as deformation preventing members are circumferentially disposed in the device body 13, in the vicinity of the through hole 15. The auxiliary pin 17 has an insert part 17a of rectangular cross section to be inserted into a water jacket 9. The auxiliary pin 17 suppresses the deformation of the low rigidity part of the cylinder bore 7 against the pressure of a grinding wheel 49.