Reeb vector field of almost contact metric structure as affine motion

Smooth manifold with almost contact metric structure (i. e., almost contact metric manifold) was considered in this paper. We used a modern version of Cartan’s method of external forms to conduct our study. We assume that its Reeb vector field is affine motion. We got formulas for components of second covariant differential of contact form for an arbi­trary almost contact metric manifold. Criterion for affine motion of Reeb vector field has been obtained for arbitrary almost contact metric mani­fold in this paper. It is proved that if Reeb vector field of almost contact structure is affine motion then sixth structural tensor of almost contact metric structure is vanishing. It is proved that if Reeb vector field is affine motion and torse-forming vector field then Reeb vector field is Killing vector field. It is proved that if Reeb vector field of almost contact metric structure is torse-forming vector field and it is not Killing vector field then it is not affine motion.