A flow velocity measuring device is able to conduct a fluid to a sensor element after removal of dust and dirt contained in the fluid to accurately measure the flow velocity of the fluid even when the fluid is low in flow velocity and the device is mounted in any direction. A flow passage is formed by internal spaces, which have curved wall surfaces, introduction passages connected to the internal spaces at one ends of the wall surfaces tangentially to the wall surfaces, discharge passages connected to the internal spaces at the other ends of the wall surfaces tangentially to the wall surfaces, and branch discharge passages connected to the internal spaces at substantially right angle to a direction, in which the wall surfaces are curved, and a sensor element is arranged in the branch passage or a flow passage connected to the branch passage.