A redox flow battery cell comprising: a cell unit in which a membrane is sandwiched between a positive electrode and a negative electrode and the electrodes (104, 105) are sandwiched between a pair of cell frames (2, 3), the cell frames (2, 3) each including a frame body (22, 32) that has a penetrating window and a bipolar plate (21, 31) that contacts the electrode (104, 105) by filling the penetrating window, the frame body (22, 32) including a liquid supplying manifold (123, 124) that penetrates through the frame body (22, 32) in a thickness direction thereof to serve as a supply channel of an electrolyte, a liquid discharging manifold (125, 126) that serves as a discharge channel of the electrolyte, an inlet slit (123s, 124s) through which the electrolyte is introduced from the liquid supplying manifold (123, 124) to the electrode (104, 105), and an outlet slit (125s, 126s) through which the electrolyte is discharged from the electrode (104, 105) to the liquid discharging manifold (125, 126), wherein the frame body (22, 32) includes an inner peripheral recessed portion (22c, 33c) formed by reducing a thickness of a peripheral portion that surrounds an entire perimeter of the penetrating window so that the peripheral portion has a smaller thickness than other portions of the frame body (22, 32), the bipolar plate (21, 31) includes an outer peripheral engaging portion that engages with the inner peripheral recessed portion (22c, 33c), the outer peripheral engaging portion being a portion having a particular width and extending throughout an entire outer periphery of the bipolar plate (21, 31), and the battery cell includes a disrupting structure that disrupts a leak channel that serves as an escape route for the electrolyte, the leak channel causing the inlet slit (123s, 124s) and the outlet slit (125s, 126s) to be in communication with each other and being formed between an outer periphery of the inner peripheral recessed portion (22c, 33c) and an outer periphery of the outer peripheral engaging portion when the cell frames (2, 3) are viewed from front, characterised in that the outer peripheral engaging portion is formed to have a smaller thickness than other portions of the bipolar plate (21, 31). a cell unit in which a membrane is sandwiched between a positive electrode and a negative electrode and the electrodes (104, 105) are sandwiched between a pair of cell frames (2, 3), the cell frames (2, 3) each including a frame body (22, 32) that has a penetrating window and a bipolar plate (21, 31) that contacts the electrode (104, 105) by filling the penetrating window, the frame body (22, 32) including a liquid supplying manifold (123, 124) that penetrates through the frame body (22, 32) in a thickness direction thereof to serve as a supply channel of an electrolyte, a liquid discharging manifold (125, 126) that serves as a discharge channel of the electrolyte, an inlet slit (123s, 124s) through which the electrolyte is introduced from the liquid supplying manifold (123, 124) to the electrode (104, 105), and an outlet slit (125s, 126s) through which the electrolyte is discharged from the electrode (104, 105) to the liquid discharging manifold (125, 126), wherein the frame body (22, 32) includes an inner peripheral recessed portion (22c, 33c) formed by reducing a thickness of a peripheral portion that surrounds an entire perimeter of the penetrating window so that the peripheral portion has a smaller thickness than other portions of the frame body (22, 32), the bipolar plate (21, 31) includes an outer peripheral engaging portion that engages with the inner peripheral recessed portion (22c, 33c), the outer peripheral engaging portion being a portion having a particular width and extending throughout an entire outer periphery of the bipolar plate (21, 31), and the battery cell includes a disrupting structure that disrupts a leak channel that serves as an escape route for the electrolyte, the leak channel causing the inlet slit (123s, 124s) and the outlet slit (125s, 126s) to be in communication with each other and being formed between an outer periphery of the inner peripheral recessed portion (22c, 33c) and an outer periphery of the outer peripheral engaging portion when the cell frames (2, 3) are viewed from front, characterised in that the outer peripheral engaging portion is formed to have a smaller thickness than other portions of the bipolar plate (21, 31).