To provide a device for separating blood components facilitating collection of leukocyte components of the blood, allowing even an unskilled person to efficiently perform the collection thereof and capable of improving a collection rate of the leukocyte components, and to provide a method of separating the blood components.

A device 1 for separating blood components includes a blood storage part 2 for storing blood and a capacity adjusting mechanism 3 whereby the capacity of the blood storage part 2 can be adjusted. The blood storage part 2 includes a first storage section 4, a second storage section 5, and a third storage section 6 that is formed between the first storage section 4 and the second storage section 5, and communicated to each of the first storage section 4 and the second storage section 5. The capacity adjusting mechanism 3 is a diaphragm mechanism provided at a lower-end opening of the first storage section 4. The volume of erythrocytes and the volume of blood plasma are calculated on the basis of the hematocrit value and liquid volume of the blood stored in the blood storage part 2. The capacity of the blood storage part 2 is adjusted such that a buffy coat formed after centrifugation separation can be accumulated in the third storage section 6 of the device 1 for separating blood components.