To provide an idler device for a crawler which can set a free projecting amount according to the sort of a crawler, as well as can remove and connect the crawler easily.

Numerous projection bodies 12 are aligned along the whole periphery at the outer peripheral surface and at the center in the axial direction of an idle wheel main body 11 which is formed in a disk form having the thickness in the axial direction. The projection bodies 12 are fitted in the radius direction of the idle wheel main body 11 allowable to slide freely. Positioning means to position the projection bodies 12 at a desired position in the radius direction are provided. As the positioning means, a piston 21 is provided integrally at the inner side of each projection body 12, the piston 21 is fitted liquidtight to an operating liquid chamber 22 formed in the radius direction of the idle wheel main body 11, and the piston operating liquid is fed and exhausted to the operating liquid chamber 22.