PURPOSE: To produce the title substance useful as a surfactant in high yield by reacting a compound stable to water with a compound unstable to water by gradually adding water to the mixture while mixing these both compounds under stirring.

CONSTITUTION: A compound stable to water is reacted with a compound unsta ble to water in a reactor (e.g. kneader reactor) capable of carrying out stirring in high viscous state by gradually adding water to the mixture while mixing these both compounds under stirring to produce the aimed product without using a phase-transfer catalyst. The compound expressed by formula II is pro duced from a compound expressed by formula I (m is 0 or 1; R1 is alkyl, etc.; R2 is alkylene; n is 0-30; l is 0 or 1; M is alkali metal) and glycidol using the above-mentioned method. Further, the compound expressed by formula II is also produced by reacting a compound expressed by formula III obtained from the compound expressed by formula I and epihalohydrin with a basic aqueous solution. The above-mentioned method has advantages capable of reducing the compound unstable to water and reducing load of isolation and purification of the aimed product.