PURPOSE: To simplify the input by controlling the first and the second process feed based on a machining program and calculated speed change positions of the first process and the interim finishing diameter, the machining program and the speed change positions of the second process and the final finishing diameter respectively.

CONSTITUTION: When the final finishing diameter DF and the machining allowance S are inputted via a size data input means A, an arithmetic means B calfulates the interim finishing diameter DF' and speed change positions d1Wd4 in the first and the second process based on the data of the final finishing diameter DF and the machining allowance S. A feed control means D feeds a grindstone wheel G to the first process based on the data of the speed change positions d1'Wd4' of the first process the the interiom finishing diameter DF' calculated by the arithmetic means B and the machining program stored in a program storage means C, and feeds the grindstone wheel B to the second process besed on the data of the speed change positions d1Wd4 of the second process calculated by the arithmetic means B and the final finishing diameter DF inputted by the size data input means A and the machining program. Accordingly, the input the control data can be simplified.