A flow sensor for measuring the flow rate of a fluid, comprising: a flow sensor piping main body in which the fluid flows, a flow sensor main body which is located in the direction of the fluid flow, and a bypass flow path which is located in the flow sensor piping main body in the direction of the fluid flow, wherein the flow sensor main body comprises a flow sensor unit, the flow sensor unit comprising a fixed vortex flow generating blade member, which is located to the upstream side entrance of the fluid of the flow sensor main body, and with which the vortex flow is generated, a flow measurement rotating blade member, which is located to the downstream of the vortex flow generating blade member, and which is rotated by the vortex flow generated by the vortex flow generating blade member, a bearing in which the shaft portion of the flow measurement rotating blade member is pivoted, wherein the flow sensor main body further comprises a flow rate detection portion in which flow measurement is performed by rotating of the flow measurement rotating blade member characterized in that the bypass flow path and a flow sensor main body accommodating portion are formed as through holes in the direction of the fluid flow inside a sensor accommodating portion of the flow sensor piping main body and are separated from each other by an inner wall of the sensor accommodating portion; wherein the flow sensor unit is located in the flow sensor main body accommodating portion. a flow sensor piping main body in which the fluid flows, a flow sensor main body which is located in the direction of the fluid flow, and a bypass flow path which is located in the flow sensor piping main body in the direction of the fluid flow, wherein the flow sensor main body comprises a flow sensor unit, the flow sensor unit comprising a fixed vortex flow generating blade member, which is located to the upstream side entrance of the fluid of the flow sensor main body, and with which the vortex flow is generated, a flow measurement rotating blade member, which is located to the downstream of the vortex flow generating blade member, and which is rotated by the vortex flow generated by the vortex flow generating blade member, a bearing in which the shaft portion of the flow measurement rotating blade member is pivoted, wherein the flow sensor main body further comprises a flow rate detection portion in which flow measurement is performed by rotating of the flow measurement rotating blade member characterized in that the bypass flow path and a flow sensor main body accommodating portion are formed as through holes in the direction of the fluid flow inside a sensor accommodating portion of the flow sensor piping main body and are separated from each other by an inner wall of the sensor accommodating portion; wherein the flow sensor unit is located in the flow sensor main body accommodating portion. a fixed vortex flow generating blade member, which is located to the upstream side entrance of the fluid of the flow sensor main body, and with which the vortex flow is generated, a flow measurement rotating blade member, which is located to the downstream of the vortex flow generating blade member, and which is rotated by the vortex flow generated by the vortex flow generating blade member, a bearing in which the shaft portion of the flow measurement rotating blade member is pivoted, wherein the flow sensor main body further comprises a flow rate detection portion in which flow measurement is performed by rotating of the flow measurement rotating blade member characterized in that the bypass flow path and a flow sensor main body accommodating portion are formed as through holes in the direction of the fluid flow inside a sensor accommodating portion of the flow sensor piping main body and are separated from each other by an inner wall of the sensor accommodating portion; wherein the flow sensor unit is located in the flow sensor main body accommodating portion.