To provide a salinity adjusting device for improving productivity without depending on the intuition or experience of an operator.

A salinity adjusting device is provided with fresh water piping for supplying fresh water, sea water piping for supplying sea water, sea water quantity adjusting valve 3 connected midway of the sea water piping, mixed water piping 4 for supplying mixed water of the fresh water with the sea water, detector 5 for detecting the salinity of the mixed water, and a control circuit 6 including a comparator circuit for comparing salinity obtained from the detected signal of the detector 5 with a prescribed salinity and a circuit for adjusting the valve 3.