Recent acquisitions include the first coal investment by Mitsui & Co in 10 years. It is purchasing a stake in a Mozambique mine operated by Brazil's Vale, in which the trading firm has an indirect stake.
"The biggest reason for participating in the Moatize project is to retain excellent quality metallurgical coal that is scarce globally," Tetsuya Fukuda, general manager of Mitsui's coal division, said. "With the resource supercycle, we had been not able to buy any assets."
The partnership will be welcome for Vale, which incurred a coal loss of almost $500 million in 2013, mostly from Mozambique.
Mitsui is paying $763 million to Vale for a stake in the mine and port and rail connections, and is also committed to spending $190 million to expand the mine.
Fukuda said Mitsui also had its eyes on other assets, without elaborating.
Coal prices soared from around $50 to over $200 a ton between 2005 and 2008, making mining assets expensive.
But prices have halved in three years and are back below $70 as miners invested in new production and demand stalled due to alternative fuel sources and slower growth.
The low prices are now triggering interest in buying cheap assets in anticipation of an eventual market pick-up.
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"If you are interested in buying assets - they're probably going to be more expensive in six months time from where they are today," said Michael Elliott, global mining and metals leader at consultants Ernst & Young.
"So it's an opportunity - can they get exposure to more volumes, potentially even better quality assets than they hold," he added.
Itochu said this month it was interested in joining bidding for the giant Tavan Tolgoi coal project after Mongolia relaunched an international tender.
"Tavan Tolgoi is one of the next big opportunities. Good quality coking coal projects definitely have scarcity value," said Richard Gannon, head of metals and mining for Deutsche Bank in Australia, when asked what Japanese firms may target next.
Joining the buying spree, Idemitsu Kosan said in November it more than doubled its stake in Indonesian thermal coal miner PT Mitrabara Adiperdana. State-owned Japan Oil, Gas and Metals National has also invested in the early stages of Australian mines in recent months.
The picture only a few months earlier looked very different.
Sumitomo announced the closure of a coal mine in Australia as part of $2.3 billion of writedowns, while a potential buyer - Hong Kong-listed Up Energy Development Group - said Marubeni was prepared to unload a stake in a Canadian coal mine for as little as $1.
Marubeni, which did not disclose financial terms, jointly paid about $1 billion for the mine in 2012, according to Thomson Reuters data.