A flow reactor that mixes a first and a second fluid, wherein the first fluid flows toward a first tip between an inner wall of a first inner tubular portion and an outer wall of a second inner tubular portion, the second fluid flows toward a second tip inside the second inner tubular portion, a third fluid having low affinity for the first and the second fluid flows toward a downstream end between an outer wall of the first inner tubular portion and an inner wall of an outer tubular portion, the second tip of the second inner tubular portion does not protrude further to the side of the downstream end than the first tip of the first inner tubular portion, and the first tip is located closer to the side of the upstream end than the downstream end when viewed from a direction orthogonal to the flow direction.