A seat slide adjuster suited to weight-saving has a structure in which two rows of engaged portions are provided at a predetermined interval in a direction orthogonal to a longitudinal direction of a locking plate that constitutes a locking mechanism, and the engaged portions engage with locking tooth parts on both sides of a lower rail during locking. Force that works between the locking plate and the lower rail acts on the locking tooth parts on both sides in a well-balanced manner and is not biased to either side. As a result, the durability of the components thereof can be improved. In addition, because two rows of hole portions in the locking plate engage, rattling of the locking plate is not likely to occur.