PURPOSE: To simply and easily perform program development or the like by directly connecting a full keyboard to a serial interface part through a connection adapter provided with a level converting part.

CONSTITUTION: For example, a full keyboard (c) is directly connected to the serial interface part aa of a portable electronic apparatus (a) through a connection adapter (b) and program data for the portable electronic apparatus (a) is inputted from the full keyboard (c). Then, this program data is successively inputted to the portable electronic apparatus (a) through the connection adapter (b). In such a case, a level converting part bb of the connection adapter (b) performs level conversion to match the TTL level of the full keyboard (c) to the CMOS level of the portable electronic apparatus (a). Consequently, the full keyboard and the portable electronic apparatus are connected with the private adapter. Thus, program development or the like is simply and easily performed.