A manufacturing method of an isocyanate compound including obtaining a reaction solution containing an aldehyde compound by reacting a compound represented by Formula (a1) or (a2) with hydrogen and carbon monoxide in the presence of a metal compound of groups 8 to 10 and a phosphorus compound, neutralizing the reaction solution, purifying an aldehyde compound by distilling the neutralized reaction solution, reacting the aldehyde compound with ammonia and with hydrogen in the presence of a catalyst to obtain an amine compound; and reacting the amine compound with a carbonylating agent to obtain an isocyanate compound, wherein the phosphorus compound is represented by the Formula (R1O)3P, and the base compound is at least one selected from carbonate and hydrogen carbonate of metals of group I and carbonate and hydrogen carbonate of metals of group II, wherein the neutralizing the reaction solution is performed within a temperature range of 40° C. to 50° C.