Provided is a process for producing an isocoumarin-3-yl-acetic acid derivative, characterized by reacting a homophthalic acid derivative represented by formula (a): (wherein Rc, R2 and R are mainly protecting groups) with a malonic acid derivating represented by formula (IV): (wherein R is an organic group including a lower alkyl group; R3 is a protecting group for a carboxyl group; and X is a halogen atom or a -OM group (wherein M is alkaline metal or alkaline earth metal)). According to the above process, various isocoumarin-3-yl-acetic acid derivatives can efficiently be provided.