To provide an inline continuous mixing apparatus capable of performing, stable and continuous mixing for long time even when reactive chemical solutions are mixed.

An oil phase solution injected from an upper stream side injection port 20 forms a stream from the upper stream side injection port 20 to a discharge port 24 in a stirring passage 26 and a rotation blade 32 is provided at the down stream side from a mechanical seal 30. A polyisocyanate and a polyol injected from an injection port 22 located at a downstream side from the mechanical seal 30 can have resistance to back flow into the mechanical seal 30 even when the turbulence of stream by the swirling of the rotation blade 32 is produced. Thus, most of the reaction product is discharged from the discharge port without reaching the mechanical seal 30 even if a hard reaction product is produced by mixing chemical solutions with each other when the reactive polyisocyanate and polyol are injected from the down stream side injection port 22. Thereby, the function of the mechanical seal 30 is not reduced because the reaction produce does not attach to the mechanical seal 30.