Provided is a novel protein phosphatase inhibitor. The protein phosphatase inhibitor contains, as an active ingredient, a silicon compound represented by the following general formula (1) or a salt thereof, wherein R1, R2 and R3 are the same or different and represent a hydrocarbon group having 1 to 12 carbon atoms; X represents an optionally substituted hydrocarbon group having 3 to 36 carbon atoms or an optionally substituted heterocyclic group; and n represents an integer of 0 or 1.