To simplify operations to correct the corresponding relationship of both outputs of a tilt sensor and a tilt setter after exchanging the tilt sensor.

A design reference line Z0 which is a relationship of an output value of a tilt sensor with an output value of a tilt setter at the time of design when the tilt sensor is kept horizontal and an indicator of the tilt setter is adjusted to a horizontal reference mark and a design reference value A0 in both the output values are respectively stored in a memory. On the other hand, a mobile machine body is placed on a nearly horizontal reference plane and the indicator in the tilt setter is adjusted to the horizontal reference mark. An initial setting switch is then pushed down and both the left and the right sides of the mobile machine body are set at the position of the lower limit in the pressed down state of the initial setting switch. The difference values (a) and b between the installation reference value A1 and the design reference value A0 in both the output values of the tilt sensor and the tilt setter are then computed and the installation reference line Z1 obtained by deforming the design reference line Z0 stored in the memory so as to shift to the design reference value AO0 to the installation reference value A1 based on the difference values (a) and (b) is stored in the momory.