numerals 21 to 24 indicate each step in the procedure when the keys a and b are depressed at the same time 21 items of key in information 34 a 34 and 34 b 34 are stored in the pb 12 then when the key i is depressed each item of key in information 34 a 34 and 34 b 34 having been registered is regarded as on state information and is registered in the kb 13 and the count data 21a and 21b in the owb 14 are set at 34 0 34 at this time when key in information 34 j 34 is entered in the pb 12 four keys are depressed in effect accordingly the key in information 34 j 34 is not entered in the kb 13 22 then when the key a is released the key in information 34 j 34 is erased and items of key in information 34 b 34 and 34 i 34 are recognized and registered in the kb 13 and the owb 14 23 if at this time the number of key in operations is counted key in information which is accepted in effect is two items of key in information 34 b 34 and 34 i 34 accordingly the key in information 34 j 34 registered in the pb 12 is erased from the pb 12 as an assumptive key in information 24 in the embodiment described above the storing space of the owb 14 is adapted for accommodating key in information on three keys however by providing the capacity for n key information the key input operation more equivalent to n key roll over can be realized fig6 shows a key matrix constituting an input device which is the second embodiment of the.