To provide an inline mixing device capable of rapidly converging a control quantity to an optional target value at the time of starting operation, and also rapidly converging, even in case of fluctuation of the control quantity during control operation or even in a case that the target value is changed, the control quantity to the target value, and a control method thereof.

Before starting the operation, an operation quantity of a flow controller in operation start to a set target value is calculated based on characteristic data showing the characteristic of control quantity to operation quantity of the flow controller, the flow controller is operated in the operation quantity in operation start for a predetermined time from operation start, and the flow controller is automatically controlled after the lapse of the predetermined time. When deviation is increased during automatic control operation, a control constant is varied according to the magnitude of deviation, or the operation quantity is temporarily fixed to the operation quantity in operation start.