PURPOSE: To inspect logic of a signal, by providing a function for inspecting a signal level of a digital circuit, and also supplying an inspecting signal of a high level and a low level, or an inspecting signal of prescribed pulse width, by switching a switch.

CONSTITUTION: As for a double-throw slide switch 9, contacts H0, H1 or L0, L1 are selected by sliding an operation part 9a. To the contact H0, supply voltage +V of a probe is applied through a resistance R2, and the contact L0 is grounded by a wide opening clip 4. To the contacts H1, L1, an inspecting signal Ph of prescribed pulse width of a monostable multivibrator 11, and an inverted inspecting signal Pl are selected. As for a double-throw slide switch 10, one of inspecting signals Sh, Sl is selected in the level side, and is supplied to a signal terminal 14 of a hook system through a self-return type normal open switch 12. Also, the multivibrator 11 is triggered. Also, in the pulse side, the signals Ph, Pl are selected. A logic detecting circuit 15 is capable of detecting logic of a signal of a place to which a probe 5 has contacted, and displaying 16 it.