A seat slide device suitable for weight reduction is provided. The seat slide device (40) is structured such that lower rails (41) and upper rails (42) are both formed to be substantially symmetrical bilaterally across a center in a width direction, and lock mechanisms (43) are provided on both sides of the upper rails (42), and each of them can be engaged with the lower rails (41) to lock them. An entire structure combining the lower rails (41), the upper rails (42) and the lock mechanisms (43) is substantially symmetrical forward and backward as well as bilaterally, and a force applied to the lower rails (41) and the upper rails (42) can be absorbed substantially evenly forward and backward as well as left and right. Since the lock mechanisms (43) have an elastic lock member (430) in a substantially center part in a longitudinal direction, the lower rails (41) and the upper rails (42) are elastically deformable by which reduction of a biased load at a time of locking, improvement in vibration absorption characteristic, and the like are achieved, and a plate thickness of each material can be made thinner than conventional ones, thereby achieving weight reduction.