Training is not the only way around Japanese immigration restrictions. There are also students in part-time jobs, asylum seekers waiting for refugee applications to be processed — almost all are ultimately rejected — and South Americans of Japanese descent who can obtain special visas based on their ancestry.

Demand still outstrips supply. The number of working-age Japanese has been falling since the mid-1990s, a consequence of decades of low birthrates. Nationwide unemployment is just 3 percent, and in some places, jobs are simply going begging. There are three to four positions open in nursing care and construction for every person who applies, according to government surveys.

Eventually, Japan plans to lengthen the maximum time that trainees can stay in the country to five years, from three, and allow more kinds of businesses to hire them, including nursing homes and cleaning companies for offices and hotels.

Parliament approved the creation of a new agency to oversee the trainee program last year, in response to criticism over worker exploitation. Once it is in place, the plan is to bring in more workers.

Mr. Kimura and some other lawmakers want to go further, by establishing a formal guest worker system. Though it would not open a path to immigration — workers would still be expected to go home eventually — it would be more forthright than the current approach.

Business groups favor the proposal, and the Liberal Democrats approved it as party policy in May. The government of Prime Minister Shinzo Abe has not indicated whether it will follow through.

“If we want economic growth in the future, we need foreigners,” Mr. Kimura said.

Can’t Walk, Can’t Leave

Nobuya Takai, a lawyer who has represented foreign trainees in labor disputes, said the pretense that trainees are not workers contributes to problems.