A decontamination liquid supply device including measuring means arranged to measure a weight of a decontamination liquid stored in decontamination liquid storing means, a decontamination liquid supply passage with a tail end portion connected to the decontamination liquid storing means and a tip end portion connected to downstream side equipment, and delivery means which is provided midway in the decontamination liquid supply passage and is arranged to supply the decontamination liquid of the decontamination liquid storing means to the downstream side equipment, the decontamination liquid supply device being arranged to supply the decontamination liquid of a predetermined weight to the downstream side equipment via the delivery means by measuring a weight of the decontamination liquid by the measuring means, wherein the decontamination liquid supply device further includes: a gas supply passage for supplying a gas which is connected to the decontamination liquid supply passage at a side of the tail end portion from the delivery means, an on-off valve which opens and closes the gas supply passage, control means which receives a signal from the measuring means and is arranged to control the delivery means and the opening and closing of the on-off valve, characterised in that: the control means is arranged to store a housed weight value of the decontamination liquid which is housed in the decontamination liquid supply passage between a connection point at which the gas supply passage is connected to the decontamination liquid supply passage and the tail end portion, and when a weight of the decontamination liquid is reduced by a total value of the predetermined weight and the housed weight value as a result of starting supply of the decontamination liquid to the downstream side equipment in a state that the on-off valve is closed, the control means is arranged to open the on-off valve to introduce the gas to the decontamination liquid supply passage, supply the decontamination liquid at a downstream side from the connection point to the downstream side equipment, and return the decontamination liquid at an upstream side from the connection point to the decontamination liquid storing means. a gas supply passage for supplying a gas which is connected to the decontamination liquid supply passage at a side of the tail end portion from the delivery means, an on-off valve which opens and closes the gas supply passage, control means which receives a signal from the measuring means and is arranged to control the delivery means and the opening and closing of the on-off valve, characterised in that: the control means is arranged to store a housed weight value of the decontamination liquid which is housed in the decontamination liquid supply passage between a connection point at which the gas supply passage is connected to the decontamination liquid supply passage and the tail end portion, and when a weight of the decontamination liquid is reduced by a total value of the predetermined weight and the housed weight value as a result of starting supply of the decontamination liquid to the downstream side equipment in a state that the on-off valve is closed, the control means is arranged to open the on-off valve to introduce the gas to the decontamination liquid supply passage, supply the decontamination liquid at a downstream side from the connection point to the downstream side equipment, and return the decontamination liquid at an upstream side from the connection point to the decontamination liquid storing means. the control means is arranged to store a housed weight value of the decontamination liquid which is housed in the decontamination liquid supply passage between a connection point at which the gas supply passage is connected to the decontamination liquid supply passage and the tail end portion, and when a weight of the decontamination liquid is reduced by a total value of the predetermined weight and the housed weight value as a result of starting supply of the decontamination liquid to the downstream side equipment in a state that the on-off valve is closed, the control means is arranged to open the on-off valve to introduce the gas to the decontamination liquid supply passage, supply the decontamination liquid at a downstream side from the connection point to the downstream side equipment, and return the decontamination liquid at an upstream side from the connection point to the decontamination liquid storing means.