To easily detect an axial position of a blade edge of a tool having a diameter of about 50 microns, with accuracy of 1 micron or less.

This tool blade edge position detecting device includes a point light source 1, a line sensor 5, a peak rectification holding circuit, and a display unit. The point light source 1 irradiates a light sensitive surfaces of one or plural unit picture elements on the line sensor 5. A blade edge of a tool 3 is entered between the point light source 1 and the line sensor 5 from a vertical upper part. When the irradiated light is partially or fully interrupted by the blade edge of the tool 3, the photoelectrically converted output of each unit picture element is reduced corresponding to the interruption degree of the light. The display unit displays a position of the blade edge of the tool on the basis of an output signal of the peak rectification holding circuit detecting a degree of reduction.