To enable fixing in simple structure by fixing a projection part formed on the lower end of the through hole of an attaching pedestal and the edge part of the sensor pipe of nickel material by electronic beam welding.

Coils 7, 8 formed of very fine enamel coated metal wires are wound around the outer periphery of the sensor pipe 1 of nickel material held by an insulator 6 in a groove 5 to be housed in the groove 5. Both the ends 1a, 1b of the sensor pipe 1 penetrates and fixes an attaching pedestal 2, communicates with a main flow passage, a bypass flow passage is formed in parallel with the sensor pipe 1, and a rate of the flow rate Qs of the sensor pipe 1 to the flow rate of the bypass passage is set constant. Thus, the whole flow rate of the main flow passage is found by detecting the flow rate Qs of the sensor pipe 1. At this time, a projection part 20 is formed on the lower end of the through hole of the attaching pedestal 2, and the lower end face 12 of the sensor pipe 1 is matched with the lower end face 22 of the through hole in a plane to directly fix both by electronic beam welding. Thereby the sensor pipe 1 add the attaching pedestal 2 can be fixed in a simple structure.