A therapeutic fluid preparation device for preparing therapeutic fluid having a predetermined concentration by mixing a plurality of stock solutions, the device comprising: a mixing circuit configured to mix the plurality of stock solutions; a concentration measuring device having an electric conductivity sensor configured to measure the electric conductivity of the solution mixture in the mixing circuit and a temperature sensor configured to measure the temperature of the solution mixture, the concentration measuring device being configured to measure the concentration of the solution mixture in accordance with the measured electric conductivity by the electric conductivity sensor and the measured temperature by the temperature sensor; electrolyte supply means that is configured to supply an electrolyte having a known concentration through the mixing circuit with a temperature of the electrolyte being varied; wherein the device further comprises a calibration curve generating means that is configured to direct the electrolyte supply means to supply the electrolyte having the known concentration through the mixing circuit with varying temperatures, that is configured to measure the electric conductivity of the supplied electrolyte using the electric conductivity sensor, that is also configured to measure the temperature of the supplied electrolyte using the temperature sensor, and that thereby generates a calibration curve between electric conductivity and temperature; and concentration calculating means that is configured to measure the electric conductivity and the temperature of the solution mixture in the mixing circuit using the electric conductivity sensor and the temperature sensor, that is configured to determine, on the calibration curve, the electric conductivity of the electrolyte at the same temperature as the measured temperature of the solution mixture, and that is configured to calculate the concentration of the solution mixture by using the following relational expression: Electrolyte electric conductivity / Solution mixture electric conductivity = α∗ Electrolyte concentration / Solution mixture concentration, where a is a constant. a mixing circuit configured to mix the plurality of stock solutions; a concentration measuring device having an electric conductivity sensor configured to measure the electric conductivity of the solution mixture in the mixing circuit and a temperature sensor configured to measure the temperature of the solution mixture, the concentration measuring device being configured to measure the concentration of the solution mixture in accordance with the measured electric conductivity by the electric conductivity sensor and the measured temperature by the temperature sensor; electrolyte supply means that is configured to supply an electrolyte having a known concentration through the mixing circuit with a temperature of the electrolyte being varied; wherein the device further comprises a calibration curve generating means that is configured to direct the electrolyte supply means to supply the electrolyte having the known concentration through the mixing circuit with varying temperatures, that is configured to measure the electric conductivity of the supplied electrolyte using the electric conductivity sensor, that is also configured to measure the temperature of the supplied electrolyte using the temperature sensor, and that thereby generates a calibration curve between electric conductivity and temperature; and concentration calculating means that is configured to measure the electric conductivity and the temperature of the solution mixture in the mixing circuit using the electric conductivity sensor and the temperature sensor, that is configured to determine, on the calibration curve, the electric conductivity of the electrolyte at the same temperature as the measured temperature of the solution mixture, and that is configured to calculate the concentration of the solution mixture by using the following relational expression: Electrolyte electric conductivity / Solution mixture electric conductivity = α∗ Electrolyte concentration / Solution mixture concentration, where a is a constant.