PURPOSE: To make smaller an external force required for effecting inclining and erecting actions of a column member of heavy weight.

CONSTITUTION: A main column member is divided into an upper member 3 and a lower member 4, and a first receiving member 30 and a second receiving member 40 are provided on the member 3 and the member 4, respectively. A first rotary axis 38 and a second rotary axis 39 are mounted to the member 3, and the axis 38 or the axis 39 is brought into engagement with the member 40 to cause the axis 38 or the axis 39 to act as a rotary axis, and furthermore there are provided a first urging member 34 for generating nearly constant urging force, and second and third urging members 36a, 36b for generating different urging forces corresponding to erection angles.