PURPOSE: To provide a production method of a microcapsule provided with a preparation method of a diluted poyvalent amine solution, such that calcium carbonate, etc., are not stuck to and grown in the insides of process equipment such as a heat exchanger, a static mixer, a piping, etc.

CONSTITUTION: After a main oil fed from a main oil supply tank 1 is mixed with a polyisocyanate and an aqueous protective colloid solution, the mixture is emulsified with an emulsifier 4, the diluted polyvalent amine solution 5 is added thereto and the resulting solution is encapsulated in an encapsulating reaction tank 6. The diluted polyvalent amine solution 5 is obtained by mixing dilution water whose conductivity is preliminary made less than a specific value (50μS/cm) with the polyvalent amine, with a static mixer 15. The diluted polyvalent amine solution 5 may be heated after mixing or the dilution water before mixing may be used after preliminarily heating.