NOVELTY - A carbon fiber felt (2) comprises carbon-fiber-felt main component (4), and cavities (6) formed in carbon-fiber-felt main component. When cavity is formed by penetrating felt main component inside, the maximum values of length of each cavity to which cavity follows thickness direction of felt main component comprising 5-75% of thickness of felt main component, and the ratio of cavity with cross-sectional area of 0.3 mm2 or more to the cross-section orthogonal to the formation direction of cavity of felt main component is 0.5-75%.    USE - Carbon fiber felt is used as material for electrode (claimed) of redox flow-type battery e.g. sodium-sulfur secondary battery.    ADVANTAGE - The carbon fiber felt has low contact resistance, and suppresses the pressure loss during pouring. The electrode containing the carbon fiber felt has excellent liquid permeability and water permeability.    DETAILED DESCRIPTION - A carbon fiber felt comprises a carbon-fiber-felt main component, and cavities formed in carbon-fiber-felt main component without having open hole in both surfaces mutually opposing each other in the thickness direction of felt main component. When cavity is formed by penetrating felt main component inside, the maximum values of length of each cavity to which a cavity follows thickness direction of felt main component comprising 5-75% of thickness of felt main component and the ratio of cavity with cross-sectional area of 0.3 mm2 or more to the cross-section orthogonal to the formation direction of cavity of felt main component is 0.5-75%. When cavity is formed by closing gap in felt main component, the maximum values of length of each cavity to which a cavity follows the arbitrary thickness direction of felt main component comprising 5-75% of thickness of felt main component and the ratio of cavity with cross-sectional area of 0.3 mm2 or more to the cross-section in alignment with arbitrary thickness direction of felt main component is 0.5-75%. INDEPENDENT CLAIMS are included for the following:    (1) manufacture of carbon fiber felt; and    (2) electrode.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of carbon fiber felt.    Carbon fiber felt (2)    Carbon fiber-felt main component (4)    Cavity (6)    Surfaces of felt thickness direction (A,B)