PURPOSE: To simplify the construction of an attachment, by detecting an interference quantity between its control axes and giving a surplus speed of rotation to a motor to an extent only removing the interference quantity, in the case of the attachment having a mechanism whose main spindle mounted inside changes an angle by the plural rotary control axes.

CONSTITUTION: A machining attachment mounted to the spindle head of a horizontal boring machine or the like is formed so as to enable a main spindle 6 of the attachment to respectively rotate in the direction of a C-axis by the action of a servomotor 7 in a vertical plane and in the direction of a B-axis by the action of a servomotor 11 in a horizontal plane. Here an instruction, when it is input to, for instance, the C-axis, is transmitted to the servomotor 7 through a position controller 32 and a servo-amplifier 33, causing the position controller 32 and the servo-amplifier 33 to perform feedback control in accordance with outputs of a speed detector 30 and a position detector 31. Then here the B-axis is prevented from interference rotation by adding an instruction value -1/N (1/N is differential gear ratio) times the instruction value to the C-axis to or subtracting the former instruction value from an instruction value to the B-axis.