PURPOSE: To form especially easily the cross-sectional shape of each part of a side beam by plastic work and to improve the dimensional accuracy of a joining part by forming each cross-cut member of pipe used for a side beam into a prescribed shape before joining said cut members together in a state of their original cross-sectional shapes to form them into a side beam to form a truck frame.

CONSTITUTION: A pipe stock having a circular cross-sectional shape used for a side beam 10, for instance is cut into plural parts; bulged parts 13 are formed by bulging work, etc., at the side faces of the middle part of the cut pipe to be used for an intermediate member 12. A pipe member 11 of the cut pipes is provided with bulging work, etc. to be formed into a prescribed curved-beam shape; a pair of both end-members 11a, 11b obtained from the member 11, has each, one end having a circular cross-sectional shape to be joined to the member 12 and the other end having a semicircular cross-sectional shape. The member 12 is welded respectively to the members 11a, 11b at their original circular cross-sectional parts to form a side beam 10. A cross beam 20 consisting of pipes are joined to the parts 13 projecting mutually oppositely of beam 10.