To provide a sustained release capsule having an improved sealing and a smoothly movable piston for a continuous release of a therapeutic composition from the capsule.

This is a dosing device for composition for dosing a kind of composition to a ruminant, and comprises a sealed capsule (8) and at least an elastic holding arm (44) fitted to the outside of the it, the holding arm (44) has a projection (50) formed on a face facing to the capsule (8), the arm (44) is able to bend to make the projection (50) contact with the part of the capsule (8) and the part of the capsule (8) is so as to enable to pass through by the projection part (50) to form an open hole (51).