PROBLEM TO BE SOLVED: To provide a water safety plan support system which enables a control criterion calculation formula for regulating a proper control criterion to be changed in accordance with a characteristic specific to a water purification plant and a change in an operation state thereof.SOLUTION: In a water safety plan support system, an input device 7 receives input of a control criterion calculation formula for a water safety plan in accordance with a characteristic specific to a water purification plant and a change in an operation state thereof. A control criterion calculation formula database 9 registers the control criterion calculation formula. A control criterion calculation section 6 calculates and determines a control criterion using water quality process data input online from a water purification plant monitoring control system 2 through a network 3 and the control criterion calculation formula registered in the control criterion calculation formula database 9. A determination section 11 c
