A flow sensor comprising: a substrate (124, 156, 128) having a diaphragm portion (124B, 156B1, 128B) and a thick fixing portion (124A, 156A, 128A) surrounding the diaphragm portion, an electrical insulating film (113, 117) formed on a first surface of the diaphragm portion, flow velocity detecting means (112, 125) arranged on the electrical insulating film (113, 117), and a channel for a measurement target fluid, wherein the flow velocity detecting means (112, 125) comprises a heat-generating body and two temperature sensors (121 A, 121B), the temperature sensors being arranged upstream and downstream of heat-generating body in the flowing direction of fluid, characterized in that the substrate (124, 156, 128) has a recess (106, 157a, 157b) formed in a surface of the substrate (124, 156, 128), the recess (106, 157a, 157b) forming the diaphragm portion (124B, 156B1, 128B) and the thick fixing portion (124A, 156A, 128A); the diaphragm portion (124B, 156B1, 128B) is made of the same material as that of the thick fixing portion (124A, 156A, 128A); the diaphragm portion (124B, 156B1, 128B) and the thick fixing portion (124A, 156A, 128A) are integrally formed in the substrate (124, 156, 128); and a second surface of the diaphragm portion (124B, 156B1, 128B), is on an opposite side of the diaphragm portion (124B, 156B1, 128B) to the first surface on which the electrical insulating film (113, 117) is formed, and the flow velocity detecting means (112, 125) is formed on that surface of diaphragm portion (124B, 156B, 128B) which does not come into contact with the channel for the measurement target fluid. a substrate (124, 156, 128) having a diaphragm portion (124B, 156B1, 128B) and a thick fixing portion (124A, 156A, 128A) surrounding the diaphragm portion, an electrical insulating film (113, 117) formed on a first surface of the diaphragm portion, flow velocity detecting means (112, 125) arranged on the electrical insulating film (113, 117), and a channel for a measurement target fluid, wherein the flow velocity detecting means (112, 125) comprises a heat-generating body and two temperature sensors (121 A, 121B), the temperature sensors being arranged upstream and downstream of heat-generating body in the flowing direction of fluid, characterized in that the substrate (124, 156, 128) has a recess (106, 157a, 157b) formed in a surface of the substrate (124, 156, 128), the recess (106, 157a, 157b) forming the diaphragm portion (124B, 156B1, 128B) and the thick fixing portion (124A, 156A, 128A); the diaphragm portion (124B, 156B1, 128B) is made of the same material as that of the thick fixing portion (124A, 156A, 128A); the diaphragm portion (124B, 156B1, 128B) and the thick fixing portion (124A, 156A, 128A) are integrally formed in the substrate (124, 156, 128); and a second surface of the diaphragm portion (124B, 156B1, 128B), is on an opposite side of the diaphragm portion (124B, 156B1, 128B) to the first surface on which the electrical insulating film (113, 117) is formed, and the flow velocity detecting means (112, 125) is formed on that surface of diaphragm portion (124B, 156B, 128B) which does not come into contact with the channel for the measurement target fluid.