"Gyudon" beef-on-rice restaurants belonging to the Sukiya chain have become a favorite target of robbers.

The chain's outlets accounted for about 90 percent of all gyudon diners that were victims of stickups between January and September, according to the National Police Agency.

To counter the situation, Zensho Holdings Co., which runs the Sukiya chain, will assign at least two workers at every outlet by March.