A guide apparatus comprises annular rail means (24) and means (27, 28, 29) for attaching the annular rail means onto a cylindrical member (19) to be inspected by a inspecting means on the rail means. The attaching means is provided on the rail means and includes at least two radially stationary jaw means (28, 29) and at least one radially movable jaw means (27). Each of the radially stationary jaw means has a jaw surface to be abutted on an outer pe, iphery of the cylindrical member. A radial distance between the abutting surface of the radially stationary jaw means and the inner peripheral surface of the radial means is discontinuously variable.