To provide a water saving effect diagnostic system for calculating or diagnosing water saving quantity or water saving effects at the time of exchanging a first facility equipment group with a second facility equipment group before the exchange.

An annual water usage before renewal is obtained as a total 87 calculated from the product of the multiplication of the number of times of use 81, the number of people using the equipment 82, water quantity 83, the number of days 84, and the number of times of washing 85 for each of facility equipment 4 to 6 (a), and an annual water service usage fee 94 is calculated from the product of the multiplication of this total 87 by a water service charge unit price 92 (b). In the same way, annual water usage after renewal is obtained as a total 107 calculated from the product of multiplication of the number of times of use 101, the number of people using the equipment 102, water quantity 103, the number of days 104 and the number of times of washing 105 for each of the facility equipment 4 to 6 (c), and an annual water service usage fee 114 is calculated from the product of the multiplication of this total 107 by a water service charge unit price 112 (d). Then, an annual water saving effect sum is calculated from a difference obtained by subtracting the annual water service usage fee 114 after renewal from the annual water service usage fee 94 before renewal.