In a rotation speed detecting apparatus of an internal combustion engine having a rotor supported by an end part of the crankshaft, a pulser ring that has a detected body composed of recess-projection teeth and rotates integrally with the rotor, and a detector that detects the rotation speed of the crankshaft by detecting the detected body, the detected body has a detected surface opposed to the detector, and the detected surface is inclined in such a manner as to be located closer to the outside of the crankshaft in the axial direction of the crankshaft as getting more apart from an axis line of the crankshaft in the radial direction. In the detector, an axis line of the detector is inclined with respect to the axis line of the crankshaft in such a manner that the detector is opposed to the detected surface.