PURPOSE: To attain accurate measurement even when the ground is different, by a construction wherein an insulating member is interposed between a fixing part and an outer casing.

CONSTITUTION: Through holes 2d of an insulating member 2, through holes 10f of a fixing piece 10d and fixing parts 1d of a rack l are put on each other and the insulating member 2 and the fixing piece 10d are fixed to the fixing parts 1d of the rack 1 by screwing them to these parts by insulating screws 3. With the insulating member 2 interposed between the fixing parts 1d and an outer casing 10a of a measuring instrument 10, the outer casing is fixed to the fixing parts 1d of the rack 1. Since a plurality of measuring instruments 10 held with the respective outer casings 10a fixed to the fixing parts ld of the rack 1 made of metal are insulated from the fixing parts 1d by the insulating members 2 in such a fixing structure of the measuring instrument as stated above, the ground of each measuring instrument 10 is isolated from those of others and accurate measurement can be performed even in the case when apparatuses being different in the ground are measured.