Some smugglers work on a larger scale.

The Coast Guard said last month that it stopped a fishing boat loaded with about 460 pounds of gold, worth roughly $9.1 million. In a separate case in December, two men connected to a yakuza crime syndicate, the Inagawa-kai, were arrested at an airport in Okinawa after flying in from Macau with 250 pounds of gold stashed aboard a Gulfstream private jet.

Smugglers have sometimes become targets. Two who were on their way to a cash-for-gold shop in Fukuoka last year were confronted by criminals in police uniforms who relieved them of cases containing several hundred pounds of gold bars, worth $6.9 million. Six men were arrested last month in connection with the robbery.

Gold smugglers are essentially profiting from the tax law.

The police say the smuggled gold comes from places where purchases are not taxed, like Hong Kong or Macau. It is ferreted in mostly by plane, either directly or through neighboring South Korea, which shares air links with many Japanese cities. The smugglers, unlike legitimate importers, hide their cargoes from customs agents and skip an 8 percent duty on gold brought into the country.

Once in Japan, the gold is sold at pawnshoplike cash-for-gold stores. Such enterprises pay sellers, even individual ones, the value of the metal in addition to sales tax. That is the key to profit: Smugglers pocket the tax portion, netting an automatic gain of 8 percent. While the margin might sound small, for a commodity worth more than a thousand dollars an ounce, the money can quickly add up. A sales tax increase in 2014 expanded the potential for profit.

The risks, by contrast, can seem small.

While it is impossible to know how many smugglers avoid detection, specialists say most probably do. A man and a woman arrested on suspicion of gold smuggling in 2009 said they had made 56 round trips to Hong Kong and Australia, netting a profit of $1.4 million, before getting caught, according to records collected by the customs authorities.

The unlucky few who do get caught are typically charged only with evading the sales tax, an offense that carries a maximum fine of about $90,000. Once the fine is paid, the gold is usually returned, customs officials said.

Smuggling cases have multiplied. During the decade before the sales tax increase, the police arrested about 10 people on suspicion of gold smuggling-related tax evasion in a typical year, according to data compiled by the Ministry of Finance. In 2015, there were 294 arrests, a pace that continued last year, officials say, though official figures for 2016 are not yet available.