An apparatus for detecting a longitudinal force of a tire is provided with at least one mark (3) provided in on one circumferential line (j) in a tire (2), a first sensor (5) disposed in a vehicle body side (4) and capable of detecting a passage of the mark (3), a second sensor (7) capable of detecting a rotation angle (.theta.) of an axle (6), and a computing device (8) for computing a longitudinal force (Fr) of the tire (2) on the basis of an information of the first and second sensors (5, 7). The computing device (8) sequentially stores a rotation angle (.theta.r) of the axle (6) when the mark (3) passes through the first sensor (5). Further, the computing device (8) computes a tire warp angle (.alpha.r) corresponding to a difference (.theta.o-.theta.r) between a no-load rotation angle .theta.o (a rotation angle of the axle (6) in a no-load state) stored in advance and the rotation angle .theta.r, and sequentially computes a tire longitudinal force (Fr) of a tire applied during the running from the warp angle (.alpha.r), using a previously determined relational equation F=f(.alpha.) between the warp angle (.alpha.) and the tire longitudinal force (F).