To handle a plurality of seedling boxes without assigning an exclusive operator for supplementing the seedling boxes.

This apparatus 2 for supplementing the seedling boxes is equipped with a conveyor part 5 for transporting the seedling boxes BS for supplementing in which the seedling boxes are stacked toward a seedling box housing part 3 and a seedling box transferring part 6 for transferring the seedling boxes BS for supplementing from the end on the downstream side of the conveyor part 5 onto the seedling box Bt in the topmost stage which is the seedling box B in the topmost stage in the seedling box housing part 3 when the number of the seedling boxes B in the seedling box housing part 3 is a prescribed number or below. Pot-like seedling chambers 8 are arranged in each seedling box B in a lattice form in the longitudinal and transverse directions when viewed in a plane. The seedling box transferring part 6 makes the seedling boxes BS slide and move while guiding each seedling chamber bottom 8a of the seedling box Bb in the lowermost stage which is the seedling box B in the lowermost stage of the seedling boxes BS for supplementing so as to ride on an unopening part 8c other than opening parts 8b of the seedling chambers of the seedling box Bt in the topmost stage.