The invention provides a novel peroxide scavenger comprising a phosphine compound represented by general formula [I]: wherein Z1 and Z2 each represents a cyclic group; Ar represents an arylene group; R represents an aliphatic hydrocarbon group; Y represents phosphorus (P), nitrogen (N), or bismuth (Bi); and R1, R2, and R3 each represents a cyclic group, particularly a peroxide scavenger that can scavenge peroxides such as reactive oxygen species which are generated in mitochondria upon exposure to oxidative stress and localized in mitochondria. The phosphine compound of the invention is oxidized by the peroxides localized in mitochondria to increase the fluorescence intensity, whereby the peroxides can be scavenged.