A seat slide device includes lower rails and upper rails both formed substantially symmetrical bilaterally across a center in a width direction, and lock mechanisms on both sides of the upper rails to be engaged with the lower rails to lock them. An entire structure combining the lower rails, the upper rails and the lock mechanisms is substantially symmetrical forward, backward, and bilaterally, and a force applied to the lower rails and the upper rails can be absorbed substantially evenly forward, backward, and left and right. Since the lock mechanisms have an elastic lock member in a substantially center part in a longitudinal direction, the lower rails and the upper rails are elastically deformable for reduction of a biased load at a time of locking, improvement in vibration absorption characteristic, and the like, and a plate thickness of each material can be made thinner than conventional for weight reduction.