We present a low-lag, monthly dataset of emissions of carbon dioxide from fossil fuel consumption and industrial processes in Japan, derived directly from official activity data and starting in 2007. Such estimates will be crucial for near-real-time monitoring of emissions and making swift policy responses accordingly. When aggregated, our estimates replicate the annual data published through the UNFCCC national inventory report very well, with only small discrepancies which we describe and quantify. The dataset is produced with a lag of two months and can be used to estimate the annual emissions a year ahead of the official estimates submitted to the UNFCCC, and also provides a better starting point for short-term emissions projections.