To smoothly adjust the angle of an outer frame without applying an unnatural force to means for adjusting the angle.

A pair of guide members 17 and 17 are disposed in an upper frame rod 13 of the outer frame A. Shaft parts 19 rearward projecting by a prescribed length are projectingly provided in the respective guide members 17. A pair of mounting members 22 and 22 are disposed in the upper plate part 21 of an installation frame 11. Inclination adjusting members 26 are turnably pivoted to the respective mounting members. Hole parts 32 for inserting the shaft parts 19 of the guide members 17 are formed in the inclination adjusting members 26. The angle of the outer frame A is adjusted with the shaft parts 19 of the guide members 17 inserted in the hole parts 32 of the inclination adjusting members 26. Then, the inclination adjusting members 26 are turned relative to the mounting members 22, so that the angle can be smoothly adjusted without applying the unnatural force to the guide members 17, the mounting members 22 and the inclination adjusting members 26.