PURPOSE: To use front rear wheels as balance weights for preventing surely a service car from tumble by detecting the rise of a car body with a height detecting means to controllably extend respective outriggers for a predetermined time after the detection until the front and rear wheels are floated up and then stop the outriggers.

CONSTITUTION: When outriggers are automatically protruded, a switch is operated to change over a hydraulic change-over valve through a relay circuit or the like and extend the respective front, rear, left and right outriggers 10, 11 hydraulically. When the respective outriggers 10, 11 extend and contact the ground, then a car body 1 floats up so that plate springs 7, 7 are loosened as the front and rear wheels 8, 9 contact the ground by their own weight. A chain 12 is pulled by the loosening of the plate spring 7 to turn on limit switches LS1, LS2 just before the front and rear wheels 8, 9 leave the ground. Thus, the hydraulic change-over valve is again changed over after a predetermined delay time by an action of an on-delay timer to stop the extension of the respective outriggers 10, 11.