This paper examines how Japan's 2016 expansion of employees' social insurance coverage affected women's employment and working hours. At firms with 501 or more employees, the reform lowered the prescribed weekly-hours threshold for mandatory pension and health-insurance coverage from about 30 to 20 hours. Using the Labour Force Survey's rotating panel, we study one-year transitions among women aged 45–59 initially working 20–29 nonregular hours, comparing large firms subject to the reform with smaller firms in a difference-in-differences design. Among married women—likely exempt from contributions before the reform as dependent spouses—the probability of remaining at 20–29 hours fell by 7.55 percentage points, while moves below 20 hours and to 30–39 hours rose by 4.57 and 2.66 points, respectively. Among unmarried women—more likely to have paid their own pension contributions—moves to 40–49 hours rose by 11.77 points. Reductions below 20 hours were largest among married women with university-educated or high-earning husbands. Triple-difference estimates show little change in wages or bonuses. Coverage expansion can thus shift hours in opposite directions, depending on workers' prior insurance status and household circumstances.