A flow meter includes a meter body and a pressure sensor. The meter body has a liquid impact surface, a sensing surface opposite to the liquid impact surface, and a mounting hole extending from the sensing surface toward the liquid impact surface. The mounting hole is a blind hole. The pressure sensor is mounted in the mounting hole, and has a resistance value that can be measured and that can be changed correspondingly with a change in liquid pressure caused by a change in flow rate. A device for producing an active hydroxyl free radical solution is also disclosed.