A non-contact type optical sensor has been incorporated into a system that performs positioning in the rotation direction for tool regrinding (hereinafter called phase alignment) in order to enable automatic positioning at the machining start point for small diameter tools of φ 1 mm and less, for which it was difficult to perform regrinding in the past. Up until now, a touch probe has generally been used to perform automatic positioning for tool grinding. However, a certain load must be applied through the contact stylus to activate the touch switch and in turn drive the touch probe sensor mechanism. Accordingly, when performing phase alignment for small diameter tools, the tool rake face is extremely small, and a proper contact point for the stylus cannot be provided. Furthermore, the extremely low rigidity of small diameter cutting tools makes measurement impossible due to the tendency of the tool to be easily deformed or broken. A non-contact type optical sensor has been incorporated in the phase alignment process for regrinding of small diameter tools in order to address these problems. The least squares method is used to obtain the phase angle applied to the maximum tool diameter from the measured data, and it has been verified that automatic positioning that is adequate for regrinding of small diameter tools can be performed.