A communication apparatus which realizes communication flow control which is capable of preventing decrease in reception performance without causing disappearance of communication data. A first storage unit stores data transmitted from an external apparatus. A second storage unit stores data transferred from the first storage unit. A determination unit determines, in a case of executing communication with the external apparatus, determine a notification size to be notified to the external apparatus as a size of data that can be received by the communication apparatus on a basis of a size of a free space of the first storage unit and a size of a free space of the second storage unit. A notification unit notifies the notification size determined by the determination unit to the external apparatus.