Flow sensor having a flow indicator having a plurality of light emission sections

A flow sensor comprises a detection section and a main unit section. The detection section comprises a casing, a through water pipe line, a transmitter which transmits an ultrasonic wave, a receiver which receives the ultrasonic wave from the transmitter, and a flow indicator having light emission sections. The main unit section comprises a display section for displaying the flow quantity value. The casing and the through water pipe line have a width perpendicular to the longitudinal direction of the through water pipe line, and the width of the casing is approximately equal to the width of the through water pipe line. One of the faces of the casing consists of the light emitting sections and a planar face whereby the width of the casing can be minimized. The detection section further comprises an alarm detector and a compute unit performs different processing when the alarm signal is on.