The present invention provides a method for the preparation of N-substituted carbamic acid ester and a method for the preparation of isocyanate using the N-substituted carbamic acid ester. According to the method for the preparation of N-substituted carbamic acid ester, the N-substituted carbamic acid ester is prepared by organic amine, carbonic acid derivative and hydroxy composition which comprises one or more than two hydroxy compounds, wherein the method comprises a step (a) and a step (b). In the step (a), reaction between the organic amine and the carbonic acid derivative is caused for obtaining a reaction mixture which comprises a compound with carbamido. In the step (b), an urethane manufacturing reactor with a condenser is used for causing reaction between the compound with carbamido and the hydroxy compounds for manufacturing the N-substituted carbamic acid ester, wherein, the hydroxy composition and a compound having a carbonyl group derived from the carbonic acid derivative by introducing the hydroxy composition, the compound having a carbonyl group derived from the carbonic acid derivative, and air containing ammonia produced as a by-product in the reaction are introduced into the condenser which is equipped on the urethane manufacturing reactor, thereby condensing the hydroxy composition and the compound having the carbonyl group derived from the carbonic acid derivative.