A chemical solution injection system, comprising: a chemical solution reservoir which stores a chemical solution containing a depositing substance to deposit to a member in contact with cooling water circulating in a plant and having a lower temperature than the cooling water has; a pipe through which the chemical solution supplied from the chemical solution reservoir passes; a plurality of heat exchangers (33a, 33b, 33c, 33d) adapted to exchange heat between the cooling water and the chemical solution passing through the pipe; a cooling water pipe which connects the heat exchangers (33a, 33b, 33c, 33d) in series and through which the cooling water guided from an inside of the plant flows; and a pump adapted to convey the chemical solution through the pipe, wherein both ends of each of the plurality of heat exchangers (33a, 33b, 33c, 33d) into and from which the chemical solution flows are provided with valves (51, 52, 53, 54, 55, 56, 57, 58) subjected to open and close control, and the chemical solution passes through the plurality of heat exchangers (33a, 33b, 33c, 33d) in series or in parallel according to setting of the valves (51, 52, 53, 54, 55, 56, 57, 58) such that the chemical solution flows in an order of the chemical solution reservoir, the heat exchanger on an upstream side, the heat exchanger on a downstream side, and the plant, and that the cooling water passes through in an order of the heat exchanger on the downstream side and the heat exchanger on the upstream side while heating the chemical solution to reduce a difference in temperature between the chemical solution and the cooling water. a chemical solution reservoir which stores a chemical solution containing a depositing substance to deposit to a member in contact with cooling water circulating in a plant and having a lower temperature than the cooling water has; a pipe through which the chemical solution supplied from the chemical solution reservoir passes; a plurality of heat exchangers (33a, 33b, 33c, 33d) adapted to exchange heat between the cooling water and the chemical solution passing through the pipe; a cooling water pipe which connects the heat exchangers (33a, 33b, 33c, 33d) in series and through which the cooling water guided from an inside of the plant flows; and a pump adapted to convey the chemical solution through the pipe, wherein both ends of each of the plurality of heat exchangers (33a, 33b, 33c, 33d) into and from which the chemical solution flows are provided with valves (51, 52, 53, 54, 55, 56, 57, 58) subjected to open and close control, and the chemical solution passes through the plurality of heat exchangers (33a, 33b, 33c, 33d) in series or in parallel according to setting of the valves (51, 52, 53, 54, 55, 56, 57, 58) such that the chemical solution flows in an order of the chemical solution reservoir, the heat exchanger on an upstream side, the heat exchanger on a downstream side, and the plant, and that the cooling water passes through in an order of the heat exchanger on the downstream side and the heat exchanger on the upstream side while heating the chemical solution to reduce a difference in temperature between the chemical solution and the cooling water.