To provide: a chemical injection device using a microorganism contamination monitor which can accurately and quickly monitor a trend of an increase of a microorganism by a relatively simple constitution; a circulation cooling system; and a membrane separation system.

A chemical injection amount is increased when there is temporally increased a potential difference or a current value of the microorganism contamination monitor which is composed of a non-conductive diaphragm 2 having proton permeability, a first conductive body (negative electrode 6) which is arranged at one face of the diaphragm 2 and with which liquid to be inspected contacts, and a second conductive body (positive electrode 5) which is arranged at the other face of the diaphragm 2 and with which an electron acceptor-containing fluid such as a ferric chloride aqueous solution contacts, and when the potential difference or the current value is temporally decreased, chemical injection control is performed so that the chemical injection amount is reduced.