With the government wrestling with statistics that indicate Japan will need as many as 30 million extra pairs of hands by 2050 to maintain current levels of economic prosperity, Hidenori Sakanaka's new book, Immigration Battle Diary, underlines his belief that the country's future will be either big or small.
"The nation has two options," says Sakanaka, who retired as head of the Tokyo Immigration Bureau in March.
"The natural decline in the population will make us a small country with a small population or, to compensate for the decline and maintain the economy, we accept more foreign nationals and become a big country."
The first option - which Sakanaka initially championed, although he is not so sure of now - would see the Japanese population fall to about 64 million people, half the present level, with robots providing the unskilled labour.
And while other countries have had few qualms about welcoming immigrants, the issue of 30 million foreigners is unlikely to sit well with traditional Japanese society, he admits.
"The problem of our declining population today is similar to the changes that were happening during the Meiji Restoration," says Sakanaka.
Between 1868 and 1912, Japan was wracked by discussions on whether it should open its doors to the Western powers.
"The debate then was whether foreigners should be excluded, and now is the time that we need to have an active debate on the same issue that is facing Japan."
Debate rages in Japan on the
dangers of Western influence
And regardless of which choice Japan makes on its society in the next 50 years, "harsh trials await us", believes Sakanaka.
"If we are to have more foreigners in Japan, then what kind of relationships will the Japanese be able to have with these other ethnic groups?" he asked. "I wonder whether Japanese people can deal with a diversified country and recognise them as equals."
A survey by the Cabinet Office last year found that 30% of Japanese people opposed making it easier for tourists to enter the country out of fear of rising crime rates.
However, the country must consider the implications of inaction. Last week, the Ministry of Health, Labour and Welfare reported that a mere 1,107,000 babies were born in 2004.
It also stated that the national birth rate had fallen to a record low of 1.28 - well below the 2.08 babies that every Japanese woman needs to produce during her lifetime to replenish the population.
At the same time, Japanese are living longer than ever, creating a vast imbalance in taxes raised on the people who work and the amount that needs to be spent on pensions and healthcare for the elderly.
"My book is balanced on the question of whether Japan should be 'big' or 'small,' and while the trend of a declining domestic population cannot be halted, I believe a small Japan would not be all bad," he said.
As the Japanese population ages,
workers' taxes get higher
"It would solve the problem of over-crowding, for one thing, but if it occurred too quickly, then it would cause serious social and cultural problems.
"In that case, we would need help from overseas or Japan would not be able to function.
"I have spoken to politicians of the Liberal Democratic Party, and they say it is a difficult debate because of public attitudes on the question of immigration," said Sakanaka. "I say that we must have this debate now."