Be careful next time you're about to shuck a fresh oyster at a restaurant — you may be using a knife considered illegal under the Firearm and Sword Control Law.

Possessing certain types of oyster knives will be punishable from Sunday, after the moratorium expires on a revision to the law that was implemented in January.

The revised law prohibits owning a double-edged knife with a 5.5 cm or longer blade and "a very sharply pointed tip." Violators face either imprisonment of up to three years or a fine of up to ¥500,000.

The law was revised in the wake of a random vehicular and stabbing rampage that killed seven in Tokyo's Akihabara district last June. Tomohiro Kato stands accused of using a double-edged dagger to stab passersby in an attack that shocked the nation.

The Hokkaido Prefectural Police announced Wednesday, only three days before the moratorium is due to end, that certain types of oyster knives will be among those banned, surprising many in eastern Hokkaido, one of Japan's largest oyster production sites.

"Workers at fish shops, fish-processing plants and restaurants have oyster knives. The town of Akkeshi (in eastern Hokkaido) is particularly famous for its production of oysters," said a police officer in the nearby city of Kushiro.

Takeshi Uei, also a Kushiro cop, said police have already received many inquiries from worried residents.