Electrodeionized water production apparatus and method for operating same

In relation to the particle sizes of an ion exchange resin, particles with a diameter of at least 0.1 mm but not more than 0.4 mm are deemed small particles, and particles with a diameter exceeding 0.4 mm are deemed large particles. In a desalination chamber (23) of an EDI apparatus (10) which removes boron from a water to be treated, a large particle layer formed from an ion exchange resin with large particles and a mixed particle layer which is a mixture of an ion exchange resin with large particles and an ion exchange resin with small particles are disposed in the direction of flow of the water to be treated so that at least one large particle layer is present on the upstream side of the mixed particle layer. At least a portion of the ion exchange resin packed in a concentration chamber (24) is a cation exchange resin.