A fixing structure of a spindle balancer for a machine tool in which a spindle balancer can be attached easily to a machine tool at an appropriate position in an appropriate posture while being supported stably and surely by the machine tool, safety is enhanced when the spindle balancer is attached to the machine tool by lessening the attaching load, set-up including replenishment of the cylinder body with compression gas and regulation of gas pressure is facilitated, and the attachment time and set-up time of the spindle balancer can be shortened. The fixing structure (1) of a spindle balancer for a machine tool comprises a flange member (30) having a supported surface (30a) and provided fixedly to the cylinder body (10) of a gas spring (2), a supporting member (40) having a flange receiving surface (40b) and a fixing hole (40c) and secured to the spindle unit (Ma) of a machine tool (M), and a plurality of fastening members (45) for fastening the flange member (30) to the supporting member (40) so as to allow the supported surface (30a) of the flange member (30) to abut against the flange receiving surface (40b) of the supporting member (40).