The present invention provides industrially preferable processes for producing a sulfone derivative useful as a herbicide and an intermediate thereof. Provided are a process for producing a compound of the formula (4), comprising the following step ii: (step ii) a step of reacting a compound of the formula (2) with a compound of the formula (3) in the presence of a base to produce the compound of the formula (4), and a process for producing a compound of the formula (5), comprising the following step iii: (step iii) a step of reacting the compound of the formula (4) with hydrogen peroxide in the presence of a metal catalyst to produce the compound of the formula (5).