Modular crawler-track unit for a forest machine having a chassis, which crawler-track unit includes - a crawler-track frame, - a sleeve axle supporting the crawler-track frame to the forest machine, - a crawler mat arranged in a circulation for distributing the surface pressure of the crawler-track unit, - at least one drive wheel mounted in bearings in the said crawler-track frame on the inside of the circulation of the said crawler mat, for rotating and supporting the said crawler mat, - at least one idler wheel mounted in bearings in the said crawler-track frame on the inside of the circulation of the said crawler mat, for supporting the said crawler mat, - support means arranged in the said crawler-track frame, on the inside of the circulation of the said crawler mat, for supporting the part of the circulation that is against the ground, which support means are arranged to conform according to the shape of the surface of the ground, in order to distribute the surface pressure of the crawler-track unit evenly, and - a drive shaft mounted in bearings in the said crawler-track frame coaxially to the sleeve axle in order to transmit power from the forest machine to the crawler-track unit, and to pivot the crawler-track unit to the chassis of the forest machine, - a shaft mounted in bearings on the crawler-track frame for pivoting the said drive wheel to the crawler-track frame at a distance from the drive shaft, and - power-transmission means fitted between the said shaft and the drive shaft for transmitting power from the drive shaft to the drive wheel, characterized in that the sleeve axle is attached to the crawler-track frame, for supporting the crawler-track frame on a pivot bearing belonging to the chassis of the forest machine. - a crawler-track frame, - a sleeve axle supporting the crawler-track frame to the forest machine, - a crawler mat arranged in a circulation for distributing the surface pressure of the crawler-track unit, - at least one drive wheel mounted in bearings in the said crawler-track frame on the inside of the circulation of the said crawler mat, for rotating and supporting the said crawler mat, - at least one idler wheel mounted in bearings in the said crawler-track frame on the inside of the circulation of the said crawler mat, for supporting the said crawler mat, - support means arranged in the said crawler-track frame, on the inside of the circulation of the said crawler mat, for supporting the part of the circulation that is against the ground, which support means are arranged to conform according to the shape of the surface of the ground, in order to distribute the surface pressure of the crawler-track unit evenly, and - a drive shaft mounted in bearings in the said crawler-track frame coaxially to the sleeve axle in order to transmit power from the forest machine to the crawler-track unit, and to pivot the crawler-track unit to the chassis of the forest machine, - a shaft mounted in bearings on the crawler-track frame for pivoting the said drive wheel to the crawler-track frame at a distance from the drive shaft, and - power-transmission means fitted between the said shaft and the drive shaft for transmitting power from the drive shaft to the drive wheel, characterized in that the sleeve axle is attached to the crawler-track frame, for supporting the crawler-track frame on a pivot bearing belonging to the chassis of the forest machine.