A flow velocity detector for detecting a flow velocity of a fluid flowing through a flow passage is provided, the flow velocity detector including a flow passage-forming section which defines the flow passage; a stress light-emitting section which is arranged at the flow passage-forming section to receive a stress having a magnitude depending on the flow velocity of the fluid and which emits a light of which amount corresponds to a change of the received stress; and a light-receiving sensor which receives the light emitted by the stress light-emitting section. The flow velocity of the fluid is detected based on an amount of the light received by the light-receiving sensor to generate a detection signal. Accordingly, the structure is simple and small-sized, and the flow velocity of the fluid can be detected at low cost.