A seat cushion airbag apparatus includes a support portion, an airbag, and a fastening portion. The support portion is arranged below the cushion portion of a conveyance seat. The airbag is arranged between the cushion portion and the support portion and at the center portion in the width direction of the conveyance seat. The fastening portion fastens the airbag to the support portion and is disposed in the lower portion of the airbag in the inflated state at a position rearward of the center in the front-rear direction.