PURPOSE: To improve the strength of a seat track device, by assembling an upper rail movably in a lower rail such that when the upward load is applied on the upper rail it is engaged with the lower rail to be supported firmly.

CONSTITUTION: An upper rail 21 secured to the lower section of a seat is assembled slidably in a lower rail 22 to be secured onto the cabin floor, while elongated projections 21a, 21b are provided on left and right shoulders and both side walls. The lower rail 22 is integrally formed with a pair of upper flanges 22a at the upper end of a member having U-shaped cross-section, while an elongated projection 22b corresponding with said projection 21b is provided on both side walls. Furthermore a roller 23 is journaled at two positions of the upper rail 21 while a ball 24 is held at both shoulders of the upper rail 21. When the upward load is applied on the upper rail 21, said projections 21b, 22b will engage to support the upper rail.