which may slide along said guide 31 between said knobs 32a and 32b the guide 31 extends axially through the cylindrical member the cylindrical member 34 is cylindrical in shape similarly to a test mandrel to make it suitable for use in the continuity test the cylindrical member is also made to be freely slidable relative to the guide 31 the continuity test is carried out to examine the state prevailing inside the pipe by moving a cylindrical member having a constant length and a diameter slightly smaller than that of the pipe inside the pipe if the cylindrical member can pass smoothly through the pipe with no obstacle encountered then it may be judged that the pipe is in a proper condition it has often experienced that the cylindrical member 34 is caught by the rust etc which may build up inside the pipe thereby making it difficult to withdraw the member in order to avoid such a state the embodiment is arranged such that knobs 32a and 32b are caused to collide with the cylindrical member to afford impact forces in the same way as that employed in the embodiment 1 the impact forces may serve to release the cylindrical member 34 from the rusts with no greater forces imposed the embodiment 3 will be described hereinbelow with reference to fig8 the derusting member 40 is formed in a disk shaped configuration having a slightly increased thickness and has the guide 41 inserted into the central portion thereof a flange 42 is formed around a circumferential surface of this derusting member 40 contrary to the previous embodiment 1 the derusting member 40 in this embodiment is constantly oriented to a upright position without being forced to incline relative to the guide 41 accordingly the flange.