The couple was handed a euro695 ($980) restaurant bill after lunch last month at a restaurant just steps away from Piazza Navona. The bill included a euro115.50 ($160) tip they say they didn't agree to, Rome police said Thursday.
The case made headlines in national newspapers, reportedly angering Mayor Gianni Alemanno. Police temporarily shut down the restaurant after the couple filed a complaint, said police official Michele Laratta.
Restaurant owner Franco Fioravanti told Italian newspapers that the couple looked satisfied when they left the Passetto restaurant, and that they ordered a lavish meal that included oysters, lobsters, sea bass and porcini mushrooms.
"If one wants to spend money, with us one can," Fioravanti told La Repubblica. "What's wrong with that?"
The couple - a 35-year-old man and a 26-year-old woman - filed their complaint a few days after their June 19 lunch at the posh century-old restaurant, Laratta said.
According to the couple's account to police, they sat outdoors and an English-speaking waiter offered to bring a few dishes without the couple's consulting the menu, Laratta said.
What followed was a euro100 ($140) bottle of Sauvignon and a several-course extravaganza. Several national papers printed what they said was the receipt from the meal, which also showed pasta dishes for euro200 ($280).
After the complaint, police checked the prices on the receipt against those on the menu the Japanese said they never saw, and found a major discrepancy. "They are way higher, quite a bit disproportionate," Laratta said.
Police also sent health inspectors, who found defective refrigerators, contamination among different foods and other below-standard procedures in the restaurant's kitchens, according to Laratta and news reports.
The restaurant was closed temporarily, and officials are also considering revoking the restaurant's license.
The Passetto restaurant was closed to the public Thursday and personnel inside refused comment to the AP.
The restaurant is a classic of Rome cuisine. Its Web site lists among its guests movie stars past and present - from Ava Gardner to Leonardo DiCaprio - as well as Queen Elizabeth, Grace Kelly and Salvador Dali.