Bus lines 20, 21 are connected to peripheral devices 1, 2 such that the polarities of the bus lines are replaced with each other according to an attachment position, and information acquisition units 16-1, 16-2 determine an attachment position P1 if the polarities of a connector terminal 10-1 (or 10-2) and a connector terminal 11-1 (or 11-2) are plus and minus, and determines an attachment position P2 if minus and plus.