PURPOSE: To enable the rotation of stirring blade without troubles even in a reaction solution having high viscosity and to enable the stirring of the solution, by driving a stirring blade directly with a miniaturized motor via a speed reducer.

CONSTITUTION: At least a part of the vessel 12 is made of a non-magnetic and non-conductive material. A motor 13 is constructed by placing a rotor 20 in the vessel and a stator 21 outside of the vessel 12 interposing the non- magnetic and non-conductive part of the vessel 12 therebetween and placing a specific radial gap from the rotor 20. A speed reducer 14 driven by the motor 13 is placed in the vessel 12, and the stirring blades 15 for stirring the reaction solution in the vessel 12 is attached to the output shaft 27 of the speed reducer 14. The intrusion of sundry germs into the reaction solution in the vessel can be prevented, the gas-tightness of the vessel is improved and even a reaction solution having high viscosity can be stirred without trouble.