A cleaning method of a sterilization treatment line, in which an upstream side tank in which liquid product is stored and a downstream side tank are connected to each other through a first conduit, a first high-temperature heating unit for sterilizing the liquid product is provided at an intermediate portion of the first conduit, one or more stages of heating units (22, 23a) heating the liquid product in a stepwise manner are provided for the first conduit at portions between the upstream side tank and the first high-temperature heating unit, one or more stages of cooling units (24a, 25, 26) cooling the liquid product in a stepwise manner are provided for the first conduit at portions between the first high-temperature heating unit and the downstream side tank, a second conduit having one end coupled with a first coupling point of the first conduit upstream a stage of the heating units provided for the first conduit and disposed in a temperature range at which the liquid product can get burned and having another end coupled with a second coupling point of the first conduit downstream a stage of the cooling units provided for the first conduit and disposed in a temperature range at which the liquid product can get burnt is arranged in parallel with the first conduit, wherein the intermediate portion of the first conduit is defined between the first coupling point and the second coupling point, a second high-temperature heating unit for sterilizing the liquid product is provided to the second conduit, one or more stages of heating units heating the liquid product in a stepwise manner are provided for the second conduit at portions between the first coupling point and the second high-temperature heating unit, one or more stages of cooling units cooling the liquid product in a stepwise manner are provided for the second conduit at portions between the second high-temperature heating unit and the second coupling point, wherein at least one heating unit is disposed for the first conduit upstream of the first connection point, wherein at least one cooling unit (25, 26) is disposed for the first conduit downstream of the second connection point, the method comprising: continuously performing a sterilization treatment with respect to the liquid product to be transferred from the upstream side tank to the downstream side tank and a CIP treatment of the intermediate portion of the first conduit following which a positive pressure keeping treatment is performed inside the intermediate portion of the first conduit, the positive pressure keeping treatment is performed with respect to the liquid product which is transferred from the upstream side tank to the downstream side tank, the sterilization treatment and the CIP treatment are performed by switching the flow path of the liquid product between the intermediate portion of the first conduit and the second conduit. continuously performing a sterilization treatment with respect to the liquid product to be transferred from the upstream side tank to the downstream side tank and a CIP treatment of the intermediate portion of the first conduit following which a positive pressure keeping treatment is performed inside the intermediate portion of the first conduit, the positive pressure keeping treatment is performed with respect to the liquid product which is transferred from the upstream side tank to the downstream side tank, the sterilization treatment and the CIP treatment are performed by switching the flow path of the liquid product between the intermediate portion of the first conduit and the second conduit.