A dialysis fluid preparation apparatus 1 includes a water feed passage 2, an A fluid passage 3 to feed an A fluid, a B fluid passage 4 to feed a B fluid, and first to third concentration measurement means 20, 23, 27, mixes the purified water, the A fluid and the B fluid in predetermined proportions respectively to prepare a dialysis fluid, and measures the concentration of the prepared dialysis fluid by the concentration measurement means.Before preparation of the dialysis fluid, in a state in which the dialysis fluid and the A fluid are not flowed to the water feed passage, the B fluid is fed from the B fluid passage to the water feed passage so that only the B fluid is flowed to the water feed passage, the electric conductivity of the B fluid is measured by first to third detection units 20a, 23a, 27a of the first to third concentration measurement means, and the measured electric conductivity of the B fluid is compared with a previously set value. Thereby, the accuracy of the first to third concentration measurement means is diagnosed.Special reagents, devices or the like are not required, and it is possible to simply diagnose the concentration measurement apparatus.
