To provide a simple, small and light fixing structure by co-fastening of inner and outer circumference fitting members to a tubular member, capable of simultaneously fastening and fixing the members to be fixed which are fittingly provided on inner and outer circumferences of the tubular member, with only one fastening member screwed onto an end part of the tubular member.

In this fixing structure by co-fastening, a fixing nut 30 is screwed onto an outer circumference of a rear end of an end part shaft 20. A seal member 16 fittingly provided on the outer circumference of the end part shaft 20 is directly pressed by a front end part of the fixing nut 30, while a bearing outer ring 13A fitted in an inner circumference of the end part shaft 20 is pressed by the front end part of the fixing nut 30 through a pressure adjusting ring 40, i.e., a fastening force adjusting member. In this state, the fixing nut 30 is fixed by a fixing ring 50 provided toward a rear end of the fixing nut 30 so as not to be rotated, while a retaining ring 60 prevents detachment of the fixing ring 50.