PURPOSE: To facilitate a countermeasure and treatment of noise using an oscilloscope, by providing a bypass circuit for noise removal capable of operating connection and disconnection selectively to a probe itself.

CONSTITUTION: In order to observe a malfunction caused by the noise of an electric circuit or the noise of the extent that a noise component is contained, the waveform observation is carried out by bringing an electrode 1a into contact with an observing point and an electrode 2a with a reference electric potential point respectively while keeping opening a push button 13 open. On one hand, when there is the possibility of carrying out the malfunction of parts of the observing point by the noise, the condition connecting a capacitor 14 between the observing point and the refernce electric potential by using conductors 1, 2 as lead wires by pushing the button 13, is constituted. The existence of malfunction in this state is observed and when the effect of noise removal by the capacitor 14 is ascertained, a capacitor having the capacity of the same degree as the capacitor 14 is fitted between the observing point and the reference electric potential and the noise removal treatment is accomplished.