Here is a quote from the Sept. 29 article "Brace for a possible spring shock": "There isn't much ambiguity about shakai hoken: If a company employs more than five people, and an employee is working more than 20 hours a week for a period longer than two months, the company is obligated to submit paperwork for an employee's health insurance and pension to the Social Insurance Agency within five days of hiring."
I believe the writer is referring to unemployment insurance. There is nothing in Japanese law to suggest that an employee working more than "20 hours" a week has to be covered (by shakai hoken). In fact, the criteria is far more complicated and refers to 75 percent of full time. If 40 hours a week is accepted as full time, that would put the cutoff point at 30 hours.
Two years ago a reprieve was granted on a plan to introduce what the writer has suggested is now law. At that time, the government said it would give companies plenty of notice before introducing the "over 20 hours per week" criteria, which for contracted employees included a salary of more than ¥98,000 and more than a year of employment.