A flow sensor includes an auxiliary channel having an opening into which a fluid to be measured is taken; a sensor element that measures the flow of the fluid to be measured; a housing that accommodates electronic parts; and a resin cover. The flow sensor is configured such that junctions of the housing and the cover are formed in locations where first target weld portions, which are formed so that the circuit chamber is surrounded, face each other and second target weld portions, which are disposed for additional reinforcement of the joints, face each other on a bonding face of the housing and a bonding face of the cover with a step being provided. The positioning of the housing and the cover is determined, and the first target weld portions are welded to each other and second target weld portions are welded to each other by way of laser radiation.