PURPOSE: To completely execute live insertion and drawing out of a card for a hot-line by constituting a state deciding means so that a buffer is set to a prescribed state immediately after a power source is supplied to the buffer with a power source supply means, and the prescribed state is released at the time when the buffer is connected to a system bus.

CONSTITUTION: The card is provided with power source supply means (a), 1 for operating buffers 12 - 14 for the input of data or the output of data or the input/output of data at the time of precharge of an electronic circuit, and state deciding means 8 - 10 for setting these buffers 12 - 14 to a prescribed state immediately after the power source is supplied to the buffers 12 - 14 by these power source supplying means (a), 1, thereafter, releasing the prescribed state, when these buffers 12 - 14 are connected to a system bus 15. In such a manner, a hot-line card can be inserted and drawn out completely without exerting an undesirable influence on data on the system bus 15 of a mother board side or data inputted and outputted to other cards connected to this system bus 15.