An apparatus for locking a table of a seat back to selectively secure the table in multiple stages, may include a base, a shaft coupled to the base and the table, an actuating arm fastened to the shaft and rotatable with the shaft, the actuating arm having actuating gear teeth, a locking arm rotatably provided on the base, wherein the locking arm includes locking gear teeth and the locking gear teeth is selectively engaged with the actuating gear teeth by the actuating arm such that when the actuating arm is rotated in a forward direction, the actuating gear teeth is engaged with the locking gear teeth or pass over the locking gear teeth according to rotational degree of the actuating arm, and a locking arm holding unit co-axially coupled with the actuating arm to the base and selectively activated by the actuating arm.