PURPOSE: To discriminate whether an output voltage is good or not and detect easily the erroneous operation of a noise margin test by only switching reference voltages for output decision test and noise margin test.

CONSTITUTION: At a function test time, terminal CD of a J-KFF is made of "L" by mode set circuit 5, and Q is made of "L" whichever values terminals J, K and CP have, and erroneous operation detector circuit 11 is not operated. Each part of output voltage decision circuit 9 performs a logical operation at this time; when the tested input is "H", LED2 of output voltage display circuit 13 is extinguished; and when the tested input is "L", LED2 is lit; and when the tested input is intermediate between "H" and "L", LED2 is flashed. At a noise margin test time, reference voltages 3 and 4 are switched by mode set circuit 5, and terminal CD is made of "H". Since FFJK1 has terminal JK connected to "H", output Q is inverted only at an input CP transition time of "H"→"L", and erroneous operations are easily detected even if the pulse width of square wave noise is narrow. Then, if a noise repeat frequency is selected, LED1 is flashed, and measurement becomes easy furthermore.