To provide a dissolving device in which lowering in the dissolving efficiency due to contamination with gas is prevented by reducing the absolute quantity of gas sucked due to contamination with gas with a solute by a dissolving pump, thereby the quality of a dissolving solution being improved when the solute is sucked from the solute tank by the dissolving pump.

There are included the dissolving pump 2 sucking, mixing and dissolving the solute and a solvent by rotation of a rotation blade 23A and a solute tank 1 feeding the solute to the dissolving pump 2. While the pressure in the solute tank 1 is kept negative, the solute is sucked into the dissolving pump 2 from the inside of the solute tank 1 under a negative pressure by the suction force generated by the rotation of the rotation blade 23A of the dissolving pump 2.

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