A considerable amount of work has shown that a carbon tax combined with research subsidies may be regarded as effective policy for encouraging the spread of low-carbon technologies for the benefit of society. This paper exploits the macro­ economic approach of endogenous growth models with technological change in or­der to make a comparative assessment of the impact of such policy measures on economic growth in the US and Japan in the medium and long term. Our estimates with the micro and macro data reveal similarities among Japanese and US energy firms as regards the elasticity of the innovation production function in R&D expenditure and the probability of radical innovation. However, according to energy patent statistics, clean innovation is not as wide-spread in Japan as it is in the US. This may explain our quantitative findings of the need for a stronger reliance on a carbon tax in Japan as opposed to the US.