To develop a water stream sensor which does not generate a mulfunction due to external disturbance.

The water stream sensor 10 is provided with: a downstream side bearing member 26; an upstream side bearing member 40; and an impeller-like magnetic generation member 28 in a casing 25, and is also provided with two magnetic sensors 30, 31 at the outer circumferential part of the casing 25. The first magnetic sensor 30 and the second magnet sensor 31 are located off by 22.5° (π/8). Because of the positional relationship where, when the first magnetic sensor 30 is in a peak environment so as to be turned on, the second magnetic sensor 31 is in an environment of a transitional region so as to be switched from the on-state to the off-state or a transitional region so as to be switched from the off-state to the on-state, the first magnetic sensor 30 functions normally in an operational environment in which the second magnetic sensor 31 is unstable.