This is to provide a process for producing an alkylene oxide adduct which can continuously produce the alkylene oxide adduct with an optional size from a small size to a large size, a temperature and a pressure at the time of the addition reaction can be controlled with high precision, and as a result, the alkylene oxide adduct with high quality having no coloring and narrow molecular weight distribution can be continuously produced, and the above can be accomplished simultaneously, and such an alkylene oxide adduct. An alkylene oxide adduct was produced by using a microflow reactor having a flow passage with an inner diameter of 0.05 to 3.5 mm, continuously supplying a predetermined amount of alkylene oxide and a predetermined amount of an organic compound having an active hydrogen atom(s) to the flow passage in a liquid state, and these materials were reacted while passing therethrough under the conditions of a temperature of the flow passage of 70 to 200°C and a pressure of the supply port of the flow passage of 1 to 10 MPa.