PURPOSE: To effectively prevent deformation due to external force and thermal deformation of a partition wall at the time of machining with respect to lowering of rigidity and thermal capacity of the partition wall between cylinder bores to improve working accuracy.

CONSTITUTION: When the inner wall surface 4a defining a cylinder bore 3 of a thin thickness cylinder block 5 is honing worked, a honing head 9 is inserted into a worked cylinder bore 3 while an auxiliary pin 10 having an expanding function due to oil pressure and a foced cooling function due to cooling oil is inserted into a cylinder bore 3 neighboring to the worked cylinder bore 3, and the inner wall surface 4a defining the worked cylinder bore 3 is worked with the honing head 9. Meanwhile, a partition wall part 4b between the worked cylinder bore 3 and the cylinder bore 3 neighboring thereto is pressed and forcibly cooled from the neighboring cylinder bore 3 side with the auxiliary pin 10.