To enable a flow rate controller to perform stable flow rate control with a small error when used under reduced pressure by providing a thermal flow rate sensor part, a controller which controls a control valve according to a flow rate detected by the thermal flow rate sensor part and an inputted set flow rate, etc.

The controller 30 is provided with a detecting circuit 31 to which heating resistance bodies 24A and 24B of the sensor part are connected. The detecting circuit 31 is provided with a bridge circuit including the resistances of the heating resistance bodies 24A and 24B and outputs the voltage of current of the bridge circuit as a flow rate output signal, which is used to display a flow rate. The flow rate output signal is sent out to a comparing circuit 32. The comparing circuit 32 compares the flow rate output signal with the externally set flow rate signal and supplies their difference signal to a driving circuit 33. The driving circuit 33 drives the control valve according to the supplied difference signal and moves up and down an actuator 13 to adjust an orifice, thereby controlling the flow rate of fluid.