To provide an entrained-air control system for a headrace which can effectively control air entrained into flowing water without requiring the upsizing of existing equipment or requiring large-scale equipment.

In a combined structure wherein a vertical shaft 2 for injecting taken-in water by using a fall is connected to the headrace 1, a barrier 5 is provided in the upper section of the head race positioned on the side of the downstream of a section to which the vertical shaft 2 is connected; an exhaust pipe 6 is connected to the upper part of the headrace 1 which is positioned between the vertical shaft 2 and the barrier 5; and a protrusion 7 which is protruded into the head race is formed in the lower section of the headrace 1 which is positioned directly below and near the vertical shaft 2. An energy dissipating mechanism for reducing the depth of the penetration of water to be injected can also be provided in the vertical shaft 2.