2 and supported rotatably by the traveling device 2 in the revolving device 3 by driving the revolving motor 7 the revolving base 6 can be revolved with respect to the traveling device 2 on the revolving base 6 the working device 4 the operation part 8 and the engine 9 which is a power source are arranged the operation part 8 has various operation instruments and can operate the backhoe 1 the operation part 8 is provided in a left front part of the revolving base 6 in the operation part 8 a seat 11 is arranged at a substantially center of a cabin 10 and an operation lever device 26 see fig2 is arranged at left and right sides of the seat 11 the operation lever device 26 can operate the working device 4 and the revolving base 6 the operation part 8 has an accelerator 27 for changing a throttle opening degree of the engine 9 and a switch 28 which is a switching means see fig2 by operating the accelerator 27 an operator can change an output of the engine 9 rotational speed of the engine 9 the switch 28 selects alternatively whether later discussed low idle control is confirmed or not whether automatic deceleration control is confirmed or not or whether both the low idle control and the automatic deceleration control are confirmed or not by operating the switch 28 an operator can select whether the low idle control and the automatic.