PURPOSE: To reduce the diameter of a circular arc member to the minimum diameter, and to insert and detach it into and from a tank by enlarging an expanding and contracting diameter ratio of plural circular arc members of a radial direction expanding and contracting device for holding a circumferential shape of an outside plate of the tank.

CONSTITUTION: Plural guide grooves 2 are provided in the radial direction from the center of a disk 1, and a moving frame 3 to which a rolling roller is attached is fitted to the guide groove 2. Subsequently, one end of a connecting lever 13 is attached pivotally to each moving frame 3, and the other end is attached pivotally to an operating lever 11 which is operated so as to move forward and backward along the center shaft of the disk 1. Also, a circular arc member 4 formed to an inclined surface 9 whose both ends are expanded and opened in parallel to said center shaft, and a guide rail member 6 being in parallel to the center shaft are provided alternately on the moving frame 3. Also, an overhang circular arc member 4' having a both- end inclined surface 9' fitted in and guided to and by the both end inclined surface 9 of the circular arc member 4 by an energizing force of a compression coil spring 8 from the derection parallel to the center shaft is installed movably to the guide rail member 6. Accordingly, the split circular arc members 4, 4' move without interfering with each other at the time of expansion and contraction, and the expanding and contracting diameter ratio becomes large.