Abstract: The method for producing an aldehyde or ketone compound from a corresponding primary or secondary alcohol at relatively high temperature within a short time with a high yield including a step (1) of reacting a sulfoxide compound with an activating agent to produce an activation reaction product; a step (2) of reacting the activation reaction product with a primary or secondary alcohol to produce an alkoxysulfonium salt; and a step (3) of reacting the reaction product with a base to produce an aldehyde or ketone; wherein at least one of the steps, preferably the step (1) and step (2), are carried out by using a microreactor.