A fluid device includes: a flow main body including a side wall along a first axis and configured to flow a fluid from an inflow portion provided at one side of the first axis toward an outflow portion provided at the other side of the first axis; a plate provided at the other side of the first axis of the flow main body and having a first surface intersecting the first axis; a standing wall extending along the first axis from the first surface toward the one side of the first axis and having a length along the first axis shorter than the side wall; and an ultrasonic element disposed at an outer side of a collection region of the plate and configured to transmit an ultrasonic wave along the first axis when the standing wall and a region surrounded by the standing wall on the first surface are defined as the collection region.