PURPOSE: To reduce the mounting cubic volume of a keyboard and its cost, by detecting a continuous depression of an existing specified key of the keyboard and depression of other specified key, and executing key locking or its release.

CONSTITUTION: When a specified A key 2 is depressed continuously ≥3 times at such time intervals as a one shot circuit 4 is not reset, an output of a flag 5 goes to "1", a signal KEYSTB is outputted whenever the key 2 is depressed, outputs of flags 9W11 all attain level, and a gate 12 outputs one level. When the key 2 is depressed odd times, an output of a flag 13 is inverted to "1" from "0", an output of an invertor 21 goes to "1", and gates 23W25 are enabled. In this state, when a B key 31 designated as a cipher key is depressed, exclusive cipher codes are outputted from SWs 27W29 by an encoder 26, are sent to flags 15W17 through the gates 23W25, are latched, and a key locking signal is outputted from a gate 22 through gates 18W20. In case when the key 2 is depressed even times, the output of the flag 13 is inverted to "0" from "1", and the key locking is released.