F1 (F1-ATPase) is a highly coupled rotary molecular motor and hydrolyses three ATP molecules per turn (3 ATP/turn). Recently, we have developed femtolitre reaction chamber arrays for highly sensitive measurement of biological reactions. By combining this technique with the rotating magnetic tweezers, the coupling ratio of the reverse reaction, ATP synthesis catalysed by single F1 molecules, has been investigated. The low coupling ratio of 10% (0.3 ATP/turn), catalysed by the alpha3beta3gamma subcomplex of F1, was significantly improved to 77% (2.3 ATP/turn) after reconstitution of the epsilon subunit. This result revealed the novel function of the epsilon subunit as a coupling factor of ATP synthesis catalysed by F1. The possible mechanism for highly coupled ATP synthesis supported by the epsilon subunit is discussed.