A seat cushion extension device is provided which can slide a seat cushion via a single spring, and has high assembly efficiency and a low weight due to a simple structure. The seat cushion extension device includes: a base having first and second rack gears provided to face each other; a cushion part provided on an upper portion of the base to move forward/backward; a locking gear part combined with a shaft extending from the cushion part to the base and having a first gear provided to be engaged with the first and second rack gears; and a manipulation part combined with the cushion part to move forward/backward between the cushion part and the first gear, and having a protruding part provided on a lower surface thereof.