An auxiliary seat storage apparatus of a seat is disclosed. An auxiliary seat slides in a lower portion of a main seat in a state of being folded to be accommodated in the lower portion of the main seat such that convenience of a user and storability can be improved. Further, when the auxiliary seat is accommodated in the lower portion of the main seat, the auxiliary seat is moved forward so that a foot insertion space is secured to a rear side of the auxiliary seat in a state in which the auxiliary seat is accommodated in the lower portion of the main seat such that ease of access of a passenger sitting on a rear seat can be improved.