In eastern Japan, the Choushimaru chain said it would stop using its conveyor belts altogether after a man was filmed putting a cigarette end into a jar of pickled ginger.
Last month the Sushiro chain changed its rules with diners having to collect their own utensils and sauces from staff and an express lane conveyor belt delivering food directly to the customer.
Meanwhile the Kura Sushi chain has developed an Artificial Intelligence security system.
Some of its conveyor belts will now be equipped with sensors and cameras.
The restaurant says that if someone is caught interfering with a plate and then returning it to the belt an alert would be sent to the restaurants head office and the restaurant would be told, and it would be reported to the police.
According to Japanese broadcaster NHK, police have arrested three people over the issue.