A blood purifying device for purifying blood using a blood purifier, the blood purifying device comprising: control means configured to control a transmembrane pressure difference across a blood purification membrane of the blood purifier, function acquisition means configured to acquire a function Fn, wherein the function acquisition means includes: acquisition means configured to acquire respective solute leakage concentrations N1(Tn)... Nm(Tn) at each of the one or more predetermined times Tn for respective transmembrane pressure differences P1(Tn)... Pm(Tn) as a plurality of transmembrane pressure differences Pm, wherein m is an integer of not less than 2; and calculation means configured to obtain the function Fn from respective values of the solute leakage concentrations N1(Tn)... Nm(Tn) at each of the one or more predetermined times Tn for the respective transmembrane pressure differences P1(Tn)... Pm(Tn) acquired by the acquisition means, wherein on the basis of the function Fn given by the numerical expression: N Tn = Fn P Tn, Tn wherein n is an integer of not less than 1, and where the numerical expression shows a relationship between the transmembrane pressure difference P(Tn) and the solute leakage concentration N(Tn) of a predetermined solute in the blood at each of one or more predetermined times Tn after initiation of blood purification, the control means obtains a transmembrane pressure difference Pa(Tn) corresponding to a target solute leakage concentration Na(Tn) at each of the one or more predetermined times Tn, and controls the transmembrane pressure difference during the blood purification on the basis of the transmembrane pressure difference Pa(Tn). control means configured to control a transmembrane pressure difference across a blood purification membrane of the blood purifier, function acquisition means configured to acquire a function Fn, wherein the function acquisition means includes: acquisition means configured to acquire respective solute leakage concentrations N1(Tn)... Nm(Tn) at each of the one or more predetermined times Tn for respective transmembrane pressure differences P1(Tn)... Pm(Tn) as a plurality of transmembrane pressure differences Pm, wherein m is an integer of not less than 2; and calculation means configured to obtain the function Fn from respective values of the solute leakage concentrations N1(Tn)... Nm(Tn) at each of the one or more predetermined times Tn for the respective transmembrane pressure differences P1(Tn)... Pm(Tn) acquired by the acquisition means, acquisition means configured to acquire respective solute leakage concentrations N1(Tn)... Nm(Tn) at each of the one or more predetermined times Tn for respective transmembrane pressure differences P1(Tn)... Pm(Tn) as a plurality of transmembrane pressure differences Pm, wherein m is an integer of not less than 2; and calculation means configured to obtain the function Fn from respective values of the solute leakage concentrations N1(Tn)... Nm(Tn) at each of the one or more predetermined times Tn for the respective transmembrane pressure differences P1(Tn)... Pm(Tn) acquired by the acquisition means, wherein on the basis of the function Fn given by the numerical expression: N Tn = Fn P Tn, Tn wherein n is an integer of not less than 1, and where the numerical expression shows a relationship between the transmembrane pressure difference P(Tn) and the solute leakage concentration N(Tn) of a predetermined solute in the blood at each of one or more predetermined times Tn after initiation of blood purification, the control means obtains a transmembrane pressure difference Pa(Tn) corresponding to a target solute leakage concentration Na(Tn) at each of the one or more predetermined times Tn, and controls the transmembrane pressure difference during the blood purification on the basis of the transmembrane pressure difference Pa(Tn).