PURPOSE: To provide a cylinder block inside surface quenching method which facilitates a lump heating/quenching process upon partially splitting any desired part of the internal circumferential wall of a cylinder block.

CONSTITUTION: From a power supply, not illustrated, an AC current is supplied through a cable which is connected with cable connecting parts 5a, 5b of a heating coil 7, and an alternately varying magnetic flux is generated by this AC current in the space to the inner wall surface 12 of a cylinder block 11 which is installed as confronting the outside circumferential wall of the coil 7, wherein the intensity of the magnetic flux Φ generated by the alternately varying flux is less in a projecting part and large in each recessed part 4. Accordingly the inside circumferential wall 12 of the cylinder block 11 confronting the position where recess 4 is formed is heated more intensely within the same heating time, and it is possible to form in lump a quenching layer 12a partially in the circumferential direction of the internal circumferential wall 12 of the cylinder block 11.