A transfer point of a transfer arm is detected accurately and stably by using a position detecting wafer having an electrostatic capacitance sensor. A position detecting wafer S is formed in a wafer shape transferable by a transfer arm 20 and includes an electrostatic capacitance sensor 50 for detecting a relative position with respect to a reference object by detecting an electrostatic capacitance in relation with the reference object for a position detection. The electrostatic capacitance sensor 50 includes a detection electrode 52 for forming the electrostatic capacitance in relation with the reference object, and the detection electrode 52 is installed on a rear surface of a main body of the wafer shape. Installed on the main body is a guard electrode 100 covering the detecting electrode 52 when viewed from a front surface thereof, for blocking an electric field oriented toward the detection electrode 52 from the front surface.