As Halloween revelers prepare to take to the streets in nurse uniforms and other costumes, the Japanese Red Cross Society is warning that using the iconic red cross emblem without authorization is against the law — even as a costume accessory.

The emblem, often mistaken for a general symbol of medical care, is protected under international and domestic laws and is reserved for specific humanitarian purposes.

The Red Cross emblem was established under the Geneva Conventions to identify people and facilities that must not be attacked during armed conflicts. Japan’s law on the Red Cross emblem also prohibits anyone other than the Japanese Red Cross Society and those authorized by it from using the symbol without permission. Violators face up to six months in prison or a fine of up to ¥300,000 ($1,900).

“The Red Cross emblem is not a symbol of medical care.…