PURPOSE: To obtain a method for detecting attenuation ratio of probe unit positively even in electrically or mechanically unstable ambient state.

CONSTITUTION: The automatic detecting method comprises a step for providing a probe unit 10 having a probe connector to be fit at a position corresponding to the connecting part of the probe unit 10 and an oscilloscope body 20 with a protrusion 11 being provided on the surface contacting with the oscilloscope, and a step for providing the oscilloscope 20 having a switch for detecting the position of the protrusion 11 disposed on a circular outer frame on the surface where a probe contacts with an oscilloscope connector for fitting a connector to a predetermined position corresponding to the connecting part of the probe unit 10 and the oscilloscope body 20, wherein the type of probe is detected based on the positional relationship between the protrusion of the probe unit 10 and the switch 21 of the oscilloscope body 20 at the time of connection.