Exploiting heterogeneous variations in labour cost increases due to Japan's 2003 social insurance premium reform as a natural experiment, we estimate the impacts of the increased social insurance premiums on employment, working hours and payroll costs. Using the difference‐in‐differences method with establishment fixed effects, we find that firms reduce the number of employees and increase average annual earnings from longer working hours in response to an exogenous increase in labour costs without productivity gains. Firms manage to pay for this increase in the average wage paid to the remaining workers by reducing the number of employees to keep total payroll costs unchanged. In contrast, since social insurance premiums are shared equally between employees and employers, firms pay the remaining half premiums that they are imposed with. Sub‐sample analyses show that firms adhering to a labour hoarding policy did fire many workers taking advantage of the 2003 reform. This may indicate that the reform provided a good excuse to cut employment in firms that had been forced to comply with a labour hoarding policy even in an over‐employment situation, which is more likely in sectors and countries where dismissals are rigorously regulated.