A redox flow battery (1, 2, 3, 4, 5) charged and discharged by supply of an electrolyte in a tank (106, 107) to a battery element, comprising: an upstream pipe (11, 21) for supplying the electrolyte in said tank (106, 107) to said battery element; a downstream pipe (12, 22) for returning the electrolyte from said battery element to said tank (106, 107); and an accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) provided in said tank (106, 107) and connected to said upstream pipe (11, 21) or said downstream pipe (12, 22), a portion of an electrolyte flow path formed of said upstream pipe (11, 21), said battery element and said downstream pipe (12, 22) being a low position portion (11L, 21L) provided at a position lower than a liquid level of the electrolyte in said tank (106, 107), one end of said upstream pipe (11, 21) or said downstream pipe (12, 22) connected to one end of said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) opening at a position close to the liquid level or at space above said liquid level of the electrolyte in said tank (106, 107), the position close to the liquid level being a position greater than (L/2) and less than L from the bottom of the tank (106, 107), when the distance from the bottom of the tank (106, 107) to the liquid level of an electrolyte in the tank (106, 107) is represented as L, the other end of said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) opening at a position close to the bottom of said tank (106, 107), the position close to the bottom of said tank (106, 107) being a position equal to or less than (L/2) from the bottom of the tank (106, 107), characterised by: said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) being provided with a leakage prevention hole (10h, 20h, 30h, 40h) at a position close to the liquid level of the electrolyte in said tank (106, 107), the position close to the liquid level being a position greater than (L/2) and less than L from the bottom of the tank (106, 107), when the distance from the bottom of the tank (106, 107) to the liquid level of an electrolyte in the tank (106, 107) is represented as L, said leakage prevention hole (10h, 20h, 30h, 40h) being smaller than an opening (10o, 20o, 30o, 40o) at the other end of said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A). an upstream pipe (11, 21) for supplying the electrolyte in said tank (106, 107) to said battery element; a downstream pipe (12, 22) for returning the electrolyte from said battery element to said tank (106, 107); and an accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) provided in said tank (106, 107) and connected to said upstream pipe (11, 21) or said downstream pipe (12, 22), a portion of an electrolyte flow path formed of said upstream pipe (11, 21), said battery element and said downstream pipe (12, 22) being a low position portion (11L, 21L) provided at a position lower than a liquid level of the electrolyte in said tank (106, 107), one end of said upstream pipe (11, 21) or said downstream pipe (12, 22) connected to one end of said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) opening at a position close to the liquid level or at space above said liquid level of the electrolyte in said tank (106, 107), the position close to the liquid level being a position greater than (L/2) and less than L from the bottom of the tank (106, 107), when the distance from the bottom of the tank (106, 107) to the liquid level of an electrolyte in the tank (106, 107) is represented as L, the other end of said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) opening at a position close to the bottom of said tank (106, 107), the position close to the bottom of said tank (106, 107) being a position equal to or less than (L/2) from the bottom of the tank (106, 107), said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A) being provided with a leakage prevention hole (10h, 20h, 30h, 40h) at a position close to the liquid level of the electrolyte in said tank (106, 107), the position close to the liquid level being a position greater than (L/2) and less than L from the bottom of the tank (106, 107), when the distance from the bottom of the tank (106, 107) to the liquid level of an electrolyte in the tank (106, 107) is represented as L, said leakage prevention hole (10h, 20h, 30h, 40h) being smaller than an opening (10o, 20o, 30o, 40o) at the other end of said accommodated pipe (10A, 10B, 10C, 10D, 30A, 20A, 20B, 20C, 20D, 40A).