is maintained so long as the both terminals h and i are h at a time t 2 when the level of the terminal h becomes l the output levels of and gates 18 and 20 become l without regard to the levels of the terminals a b and j further the output levels of or gate 19 and nor gate 23 are put under the control of the levels of signals applied at the terminals i and k namely when the level of terminal i is l the level of the output terminal g varies between h and l in accordance with a level variation of the terminal k at a time t 3 when the level of the terminal i becomes h the level of the output terminal g starts to vary between l and l it goes without saying that even if the levels of the terminals a b and j change the level of the terminal g does not change in connection with this as a result when digital signals as shown in fig6 a b h i j and k are applied at the input terminals a b h i j and k of fig3 the signal wave form appearing at the output terminal g becomes as shown in fig6 g further when digital signals as shown in fig7 h l and c are applied at the input terminals h l and c respectively the signal wave form appearing at the output terminal g becomes as shown in fig7.