To provide a platelet collecting device capable of preventing the collection of excessive amounts of platelet.

This platelet collecting device 1 comprises a platelet collecting tendency judgment function for judging the increasing the decreasing tendencies of the number of collected platelets by use of at least platelet concentration related data obtained from a turbidity sensor 14 in a platelet collecting operation, a predicted collecting number of platelets or its related value after collecting the plasma containing high-concentration platelets in the platelet collecting operation; and a reducing type platelet collecting number adjusting function for reducing the platelet collecting quantity in the following platelet collecting operation when the increasing tendency is judged by the platelet collecting tendency judgment function.