A fixing structure of a spindle balancer for a machine tool is described as having a flange member having a supported surface and provided fixedly to a cylinder body of a gas spring, a supporting member having a flange receiving surface and a fixing hole, and secured to the spindle head of a machine tool, and a plurality of fastening members for fastening the flange member to the supporting member so as to allow the supported surface of the flange member to abut against the flange receiving surface of the support member.