It is intended to provide: a compound useful as an amyloid oxidation catalyst which is applicable in vivo and is applicable not only to Aβ peptides but to other amyloids; and a prophylactic or therapeutic drug for an amyloid-related disease, comprising the same. The present invention provides a benzothiazole compound represented by the following formula (1) wherein X represents a halogen atom; R1 represents an optionally substituted hydrocarbon group; R2 represents a hydrogen atom or an optionally substituted hydrocarbon group; R3 and R4 are the same or different and each represent a hydrogen atom, an optionally substituted hydrocarbon group, an alkoxy group, a halogen atom, an amino group, a nitro group, or a cyano group; R2 and R4 optionally together form an alkylene group; and R5 represents an anion.