A flow sensor may be formed by bonding a sensor chip formed with a flow rate detecting part and a flow path-forming member that is provided on the sensor chip and is formed with a flow path for a fluid flowing in the flow rate detecting part to each other on the upper surface of a substrate. The flow path-forming member may be formed by bonding a transparent first flow path forming member and a second flow path-forming member to each other. The first flow path forming member has a plate shape, and is provided with an inflow port and a outflow port for the fluid to be measured, and the second flow path forming member has a plate shape, and is provided with a through hole that forms the flow path along the flow of the fluid flowing along the flow rate detecting part.