PURPOSE: To lift an upper surface treating device adequately controlled corresponding to a change even in the case that a tilt angle is rapidly changed of a vehicle body upper surface by leaving a vehicle body upper surface position as previously detected corresponding to a relative position between a frame and a vehicle body, and setting a lift position of the upper surface treating device based on a result of detection.

CONSTITUTION: In a control means 50, based on detection positions of a lift position detecting means D3 for detecting a lift position of an upper surface treating device 2, relative position detecting means D1 and an upper surface position detecting means D2, the lift position corresponding to a relative position of the upper surface treating device 2 is set, and based on the preset lift position and the detection position of the lift position detecting means D3, a driving means is controlled so as to lift the upper surface treating device 2 along a vehicle body upper surface shape. The control means 50, in the case of lifting the upper surface treating device 2 along a vehicle body upper surface, uses a preset lift position based on an upper surface position corresponding to the relative position in the front by a predetermined distance Lu from a correction relative position corrected by a displacement amount Lo from the detection position B of the relative position detecting means D1.