as shown in table 1 in the case of the water recovery rate of 30 the supersaturation magnification reaches 21 times when the water recovery rate is 50 higher condensation is caused and supersaturation magnification reaches 59 times that is when the seed crystals are not added the water recovery rate of 30 is a limit in order not to generate scales in the desalinization unit the calculation results of the fluoride ion concentration and the calcium ion concentration in the wastewater and the supersaturation magnification of the calcium fluoride when the wastewater is treated by adding the seed crystals by means of the wastewater treatment device of the first embodiment are shown in table 2 table 2 shows a case where the fluoride ion concentration is reduced only in the pretreated water after the wastewater treatment performed by the wastewater treatment device of the first embodiment compared to in the source water as in formula 1 the solubility product of the calcium fluoride to the water is proportional to the square of f concentration if the fluoride ion concentration in the wastewater before desalination treatment is reduced as shown in table 2 generation of the scales is suppressed to the water recovery rate of 50 that is the recovery rate of the water after the desalination treatment can be improved by adding the seed crystals as in the present embodiment fig3 is a schematic view of a pretreatment unit of a wastewater treatment device related to a second embodiment the pretreatment unit 2 in the wastewater treatment device of the second.