The items were adult diapers being turned out by automated production lines that stretch for 80 m (262 ft) at a facility run by Daio Paper 3880.T in the Fujinomiya region, near the tourist landmark, Mount Fuji.
Factory overseer Naoto Sugaya said output has grown year on year, as he held a pair of the "paper pants" close to his thigh and explained their features.
"The part around the legs is very tight-fitting," he added. "When you put them on, you can use them without any worry."
As Japan's population ages, the company is pouring more resources into the growing new market. It says revenue from adult diaper sales is already double that of those meant for babies.
With births falling last year to a record low, Japan's population is likely to decline by about 30% to 87 million by 2070, with four people in every 10 aged 65 or more, estimates show.
Those figures are pushing other companies to switch focus, too.
Another diaper maker, Oji Holdings 3861.T, made headlines this year when it said it would stop making nappies for babies and instead focus on adults.
Apart from their size, diapers for adults have key differences, since customers are more discerning than the average baby, and rely on them in a wider range of situations, said Daio Paper marketing manager Kenji Nakata.
Survey results showed potential buyers found the company's products useful, he said.
"'Wow,' they said, 'If they're like this, I'd buy them, they wouldn't be embarrassing to bring home,'" Nakata added. "'If I wore these pants I could do my hobbies, go fishing, play golf, go shopping in Ginza.'"
Daio Paper has no plans to stop making nappies for babies, Nakata said, but the future of the business was clear.
"We can expect the market for adult diapers to continue to grow, and therefore we are devoting our company's resources to that market with a view to expanding these products."
Over the five years to 2027, Japan's diaper market for adults is set to grow 16% to 98.9 billion yen ($612 million), while that for babies is estimated to contract 8% to 84.6 billion yen, research firm Fuji Keizai says. ($1=161.6000 yen)
