A fitting member positioning device, that can be reliably fixed in a predetermined position relative to an installation portion, includes a first element having an insert portion received in an insertion hole, the insert portion having a first thread portion, and a second element rotatable relative to the first element and axially movable, having a second thread portion received in the first thread portion. The insert portion includes an integral annular dilating portion, while the second element includes an advancing portion for advancing into and integrally dilating the dilating portion in the circumferential direction when the second element is axially moved with rotation by the first and second thread portions being threaded together, to press the first element against an inner circumferential surface of an insertion hole to prevent movement of the first element.