In 1965 a methane-gas explosion ripped through the no. 2 shaft of the Y-san mine, killing 237 miners and seriously injuring another 150 men. This disaster was Japan's second worst post-war accident, and was remarkable for the number of errors that occurred before and after the explosion. Although an internal company inquiry found that the management of the day was correct in its actions, an independent inquiry conducted by the Fukuoka Mine Safety Commission found that there were ‘suspicions of improper behaviour’ on the part of management, but that these suspicions could not be proved beyond all doubt.

The families of the dead and injured men took the unprecedented step of filing a civil suit against the Y-san company, suing them for compensation. This case was started even though the company and the union put considerable pressure on the widows and families of the dead and the injured miners to give up the case and take a small settlement. It was a landmark case for a number of reasons, not the least of which was that the courts found that M-san, the parent company, was negligent, the first time a mining company had been put in this position. This decision established a precedent that affected the outcome of other civil suits within the Japanese legal system over recent years.

The involvement of outsiders in this case once more ensured that the widows of the men killed in the disaster had the opportunity to pursue a relatively comfortable existence. This was achieved by mounting a sustained and comprehensive offensive against the Y-san company, and through the group of widows maintaining solidarity in the face of strong-arm tactics used by the company to convince them to accept a smaller settlement.