PURPOSE: To shorten time up to uniformizing temps. in cams by executing combination of heating treatment and air cooling treatment divided into plural times in a cam shaft having less mass at the end parts than the mass at the center side in the axial direction.

CONSTITUTION: In the cam shaft 1 having an axial hole 4 fitting plural cams 3-1-3-8 in the axial direction in order, the inner diameter D1 at the end parts is made larger than the inner diameter D2 on the center side in the axial direction and the mass per unit length at the end parts is made less than the mass on the center side in the axial direction. To this cam shaft 1, voltage is impressed from a transformer 10 through a steel-made bus bars 8, ounce copper plates 9 and heating electrodes 7, and after executing the heating treatment to the cam shaft 1, the air cooling treatment is executed and each cam 3 is made to a prescribed uniform temp. Then, by executing the combination of the heating treatment and the air cooling treatment divided into plural times, energy consumption used to the heating is restrained as less as possible and also, while preventing melting of the cam shaft 1, the uniformity of temp. in each cam 3 is obtd. in the short time.