A hemodialysis system comprising: a hemodialysis executing unit including a dialyzer configured to purify blood, a blood supply channel for supplying the blood extracted from a body to the dialyzer, a blood pump provided in the blood supply channel and for delivering the blood to the dialyzer, a blood return channel for returning the blood purified by the dialyzer to the body, a dialysis fluid supply channel for supplying dialysis fluid to the dialyzer, a dialysis fluid pump provided in the dialysis fluid supply channel and for supplying the dialysis fluid to the dialyzer, and a dialysis fluid discharge channel for discharging, from the dialyzer, the dialysis fluid used for purifying the blood in the dialyzer; a body fluid volume calculating unit configured to calculate, by analyzing a mathematical model concerning urea kinetics, a body fluid volume, which is a total volume of water present in the body of a patient, from measured serum urea concentration at a start of first hemodialysis treatment and measured serum urea concentration at an end of the first hemodialysis treatment, a dialysis treatment time of the first hemodialysis treatment, a water removal volume in the first hemodialysis treatment, blood flow velocity in the first hemodialysis treatment, dialysis fluid flow velocity in the first hemodialysis treatment, and a mass-transfer area coefficient of the dialyzer used for the first hemodialysis treatment; a dialysis condition input unit with which blood flow velocity and dialysis fluid flow velocity in second hemodialysis treatment performed after the first hemodialysis treatment can be input and input values of the blood flow velocity and the dialysis fluid flow velocity can be adjusted; a dialysis dose calculating unit configured to calculate, when the blood flow velocity and the dialysis fluid flow velocity are input to the dialysis condition input unit, by analyzing the mathematical model concerning urea kinetics, a dialysis dose of the second hemodialysis treatment from the input blood flow velocity and the input dialysis fluid flow velocity, a mass-transfer area coefficient of the dialyzer used for the second hemodialysis treatment, a body fluid volume calculated by the body fluid volume calculating unit, a planned treatment time of the second hemodialysis treatment, and a planned water removal volume in the second hemodialysis treatment; and a dialysis dose display unit configured to display the hemodialysis -dose calculated by the dialysis dose calculating unit. a hemodialysis executing unit including a dialyzer configured to purify blood, a blood supply channel for supplying the blood extracted from a body to the dialyzer, a blood pump provided in the blood supply channel and for delivering the blood to the dialyzer, a blood return channel for returning the blood purified by the dialyzer to the body, a dialysis fluid supply channel for supplying dialysis fluid to the dialyzer, a dialysis fluid pump provided in the dialysis fluid supply channel and for supplying the dialysis fluid to the dialyzer, and a dialysis fluid discharge channel for discharging, from the dialyzer, the dialysis fluid used for purifying the blood in the dialyzer; a body fluid volume calculating unit configured to calculate, by analyzing a mathematical model concerning urea kinetics, a body fluid volume, which is a total volume of water present in the body of a patient, from measured serum urea concentration at a start of first hemodialysis treatment and measured serum urea concentration at an end of the first hemodialysis treatment, a dialysis treatment time of the first hemodialysis treatment, a water removal volume in the first hemodialysis treatment, blood flow velocity in the first hemodialysis treatment, dialysis fluid flow velocity in the first hemodialysis treatment, and a mass-transfer area coefficient of the dialyzer used for the first hemodialysis treatment; a dialysis condition input unit with which blood flow velocity and dialysis fluid flow velocity in second hemodialysis treatment performed after the first hemodialysis treatment can be input and input values of the blood flow velocity and the dialysis fluid flow velocity can be adjusted; a dialysis dose calculating unit configured to calculate, when the blood flow velocity and the dialysis fluid flow velocity are input to the dialysis condition input unit, by analyzing the mathematical model concerning urea kinetics, a dialysis dose of the second hemodialysis treatment from the input blood flow velocity and the input dialysis fluid flow velocity, a mass-transfer area coefficient of the dialyzer used for the second hemodialysis treatment, a body fluid volume calculated by the body fluid volume calculating unit, a planned treatment time of the second hemodialysis treatment, and a planned water removal volume in the second hemodialysis treatment; and a dialysis dose display unit configured to display the hemodialysis -dose calculated by the dialysis dose calculating unit.