A solution generating device (60) comprises: a first line (130) which mixes water and an undiluted solution A by baffles (140) while causing the same to flow in an upward direction a second line (131) which is connected to a top end of the first line (130) and which causes a first mixture of water and an undiluted solution A to flow in a downward direction a first gas discharge section (141) that discharges gas collected at a connection between the first line (130) and the second line (131) a third line (132) which mixes the first mixture and an undiluted solution B by baffles (150) while causing the same to flow in an upward direction a fourth line (133) which causes a second mixture of the first mixture and the undiluted solution B to flow in a downward direction and a second gas discharge section (151) that discharges gas collected at a connection between the third line (132) and the fourth line (133), wherein the second line (131) is provided with a sensor (142) for measuring the concentration of the first mixture.
