PURPOSE: To reduce the occupying area of a device by forming a type of four- parts link by the upper arm, one part of lower arm, connecting rod and mast intermediate section of an earth auger device and approaching and separating a leader mast to and from the rotating machine base side.

CONSTITUTION: A boom A is constituted in an articulated arm composed of an upper arm 7 and a lower arm 6, a hydraulic cylinder 10 for derricking the boom is installed on the lower arm 6, a hydraulic cylinder 11 for tilting a mast is mounted between the upper and lower arms 7, 6 and a connecting rod 8 is pin-coupled between the lower end of the hydraulic cylinder 11 and a mast intermediate section 3a and a four parts link is formed by one parts of the upper and lower arms 7, 6, the connecting rod 8 and the mast intermediate section 3a. The leader mast 3a is organized so as to be able to approach to and separate from the rotating machine base 1 side in response to the expansion and contraction of the hydraulic cylinder 11 for tilting the mast. Accordingly, an earth auger 4 can be overhauled and assembled safely and easily at a narrow job site.