A body section having a main channel through which a fluid to be measured flows and sections for shunting the fluid to be measured from the main channel, and a fluid measuring section provided removably from the body section and measuring the flow rate of the fluid to be measured flowing through the main channel based on the detection results of the fluid to be measured introduced from the shunting section. The fluid measuring section is fixed to the body section and shunt ratio data, i.e. the adjustment data about measurement processing inherent to the constitution of the body section, is registered in the fluid measuring section before adjusting measurement processing.