To learn the overhang position of an outrigger beam as high-reliability information with a simple structure.

This device is provided on an outrigger device 7 inserted with an outrigger beam 9 having a jack 11 stretchably to the side in an outrigger box 8 fixed to a vehicle body, and it detects the overhang position of the outrigger beam 9. This device is constituted of a perforated plate 13 provided on the outrigger beam 9 and having many holes 14 formed at constant intervals along the overhang direction of the outrigger beam 9, the first and second sensors 15, 16 provided on the outrigger box 8, turned on or off by the movement of the holes of the perforated plate 13 as the outrigger beam 9 is expanded or shrunk, and generating phase signals when they are turned on or off, and a control device calculating the overhang position of the outrigger beam 9 based on the phase signals of the first and second sensors 15, 16.