A controller for controlling a synchronized operation of spindle and feed axes. A spindle-axis control section includes an initial-motion control section for accelerating a spindle axis at maximum capacity from a starting position; a maximum-acceleration detecting section for detecting a maximum acceleration of the spindle axis; a residual rotation-amount detecting section for detecting a residual rotation amount of the spindle axis; a current-speed detecting section for detecting a current speed of the spindle axis; a decelerating-motion control section for decelerating the spindle axis to reach an intermediate speed, after the acceleration; a positioning-motion control section for decelerating the spindle axis to reach the target position after reaching the intermediate speed; a flux-content predicting section for predicting an actual motor-flux content at the intermediate speed; and a deceleration determining section for determining a deceleration for a positioning operation, based on the maximum acceleration, the motor-flux-content command value and the motor-flux content.