The application relates to a seat cushion extension apparatus for a vehicle seat which includes an extension unit slidable from a front end portion of a seat frame in a vehicle to a position disposed forwards of the front end portion of the seat frame, locking units for locking the extension unit with respect to the seat frame at a locking position, and allowing the extension unit to be slidable such that the extension unit protrudes forwards of the seat frame at a release position, and a lever for controlling the locking units between the locking position and the release position. The extension unit is supported by guide shafts to be movable forwards and rearwards in housings fixed to the seat frame. The guide shafts are locked in the housings at the locking position and are movable with respect to the seat frame at the release position.