PROBLEM TO BE SOLVED: To provide a water flow detector which can, on an actuation of a pressure switch, determine whether the actuation being caused by water leak due to partial opening of a valve body or it being caused by the valve body opening due to a fire.SOLUTION: A water flow detector comprises: a valve body which closes during monitoring and opens by the pressure within a secondary-side piping becoming lower than the pressure within a primary-side piping in the event of a fire; a valve seat on which the valve body is seated; a connection pipe connected from the valve seat to a drain pipe; an auto-drip provided in the connection pipe; a branch pipe branched from the primary-side connection pipe of the auto-drip; and a pressure switch connected to the branch pipe. In the water flow detector, the pressure switch has a first and a second contact point, and the first contact point is actuated at a lower pressure than the second contact point.
