To provide a platelet collecting device hardly causing a unit shortage or unit excess resulted from a platelet recovery ratio stored in the platelet collecting device.

This platelet collecting device 1 comprises a blood treatment quantity arithmetic function for calculating the blood treatment quantity in a platelet collecting operation by use of the operation quantity related data of a feeding pump 11 for blood collection in the platelet collecting operation; an input part of the number of platelets collected from a blood offerer by the platelet collecting operation; a platelet recovery rate arithmetic function for calculating the actual platelet collecting rate from platelet concentration data before blood collection, the calculated blood treatment quantity and the number collected platelets; a storage function thereof; a rewriting platelet recovery rate arithmetic function which is operated after the storage of a prescribed number or more of actual platelet recovery rate to calculate a rewriting platelet recovery rate from the stored actual platelet recovery rate; and a platelet recovery rate memory value updating function.