To provide an outrigger device for a truck capable of being locked by only turning an outrigger arm and certainly and easily performing unlocking.

In the outrigger device for the truck, a cylinder pipe 54 provided on a base part of the outrigger arm 53 is arranged/supported on a bracket 51 fixed onto a frame 2a of the truck 2 through a bolt shaft 52, and a lock mechanism 61 for locking returning operation of the outrigger arm 53 rotated making the cylinder pipe 54 as a center and projected to the outside of the truck 2 is provided. The lock mechanism 61 is provided with a lever 62 extending from the cylinder pipe 54 in a radial direction; a weight 63 turnably supported on a predetermined portion of the lever 62; and a weight receiving surface 51c slid with the weight 63 accompanying with turning operation of the outrigger arm 53. When a slide-contact point P1 of the weight 63 and the weight receiving surface 51c is intruded to a bolt shaft 54 side than a tangent L2, it becomes in the lock state and the outrigger arm 53 cannot be lifted.