To obtain a water purifier by which the replacement frequency of a battery is reduced and the stoppage of water supply in a clogged cartridge is rapidly signaled.

This water purifier is provided with a pressure sensor 8 fixed to a first water passage 6 and detecting the pressure of the introduced raw water a water purifying part 7 receiving the raw water from the first water passage 6 and purifying it, a purified water discharge pipe 11, a flow sensor 10 driven by a battery as the power source 14 and detecting the flow rate of the purified water produced in the water purifying part 7, a lamp 13a signaling the production of purified water in the water purifying part 7 when the flow rate of purified water detected by the sensor 10 exceeds a specified level and a control means 12 stopping the power supply to the flow sensor 10 from the power source 14 when the actuation of the pressure sensor 8 is stopped.