Diagnostic method for a dialysis fluid preparation device , the latter comprising: a water supply channel for supplying water, an A-fluid channel for supplying an A-fluid containing sodium chloride as a main ingredient, together with at least one other ingredient, the A-fluid channel being connected to the water supply channel , a B-fluid channel for supplying a B-fluid consisting of an aqueous sodium bicarbonate solution, with the B-fluid channel being connected to the water supply channel , and concentration measuring means , provided with a detection unit downstream of the connection positions of the A-fluid channel on the water supply channel , the diagnostic method comprising: mixing the water, the A-fluid and the B-fluid, each in predetermined proportions, in order to prepare a dialysis fluid, and measuring a concentration of the prepared dialysis fluid using the concentration measuring means , characterized by, prior to the preparation of the dialysis fluid, supplying the A-fluid from the A-fluid channel to the water supply channel such that it is only A-fluid flowing in the water supply channel , and diagnosing a measurement accuracy of the concentration measuring means by measuring an electrical conductivity of the A-fluid, be it undiluted or diluted, using the concentration measuring means and subsequently comparing the measured electrical conductivity of the A-fluid or the undiluted A-fluid with a value set in advance. a water supply channel for supplying water, an A-fluid channel for supplying an A-fluid containing sodium chloride as a main ingredient, together with at least one other ingredient, the A-fluid channel being connected to the water supply channel , a B-fluid channel for supplying a B-fluid consisting of an aqueous sodium bicarbonate solution, with the B-fluid channel being connected to the water supply channel , and concentration measuring means , provided with a detection unit downstream of the connection positions of the A-fluid channel on the water supply channel , the diagnostic method comprising: mixing the water, the A-fluid and the B-fluid, each in predetermined proportions, in order to prepare a dialysis fluid, and measuring a concentration of the prepared dialysis fluid using the concentration measuring means , characterized by, prior to the preparation of the dialysis fluid, supplying the A-fluid from the A-fluid channel to the water supply channel such that it is only A-fluid flowing in the water supply channel , and diagnosing a measurement accuracy of the concentration measuring means by measuring an electrical conductivity of the A-fluid, be it undiluted or diluted, using the concentration measuring means and subsequently comparing the measured electrical conductivity of the A-fluid or the undiluted A-fluid with a value set in advance. mixing the water, the A-fluid and the B-fluid, each in predetermined proportions, in order to prepare a dialysis fluid, and measuring a concentration of the prepared dialysis fluid using the concentration measuring means , characterized by, prior to the preparation of the dialysis fluid, supplying the A-fluid from the A-fluid channel to the water supply channel such that it is only A-fluid flowing in the water supply channel , and diagnosing a measurement accuracy of the concentration measuring means by measuring an electrical conductivity of the A-fluid, be it undiluted or diluted, using the concentration measuring means and subsequently comparing the measured electrical conductivity of the A-fluid or the undiluted A-fluid with a value set in advance. <EOS>