To enable miniaturization of a device its cleaning.

When measuring the flow rate of a fluid or a gas flowing through a pipe 16 where two heat conduction parts 17 and 20 away from a heat source by the heat source, at least one heat conduction part is a wall (a heat conduction part 17) of a pipe 16 located at upstream or downstream side for a heat source and temperature is compared with that of the other heat conduction part 20. When measuring flow rate due to temperature difference, two heat conduction parts 17 and 20 are preferably extended in nearly parallel by a specific distance each and the edge part of the pipe 16 where fluid leaves is heated to a specific temperature. Then, a temperature curved being used is measured at the total length between both edges of the pipe 16 and another part (the heat conduction part 20) and flow rate is determined according to a region between two curves as a function of length.