  Big bang nucleosynthesis in a modified gravity model of $f(R)\propto R^n$ is
investigated. The only free parameter of the model is a power-law index $n$. We
find cosmological solutions in a parameter region of $1< n \leq
(4+\sqrt{6})/5$. We calculate abundances of $^4$He, D, $^3$He, $^7$Li, and
$^6$Li during big bang nucleosynthesis. We compare the results with the latest
observational data. It is then found that the power-law index is constrained to
be $(n-1)=(-0.86\pm 1.19)\times 10^{-4}$ (95 % C.L.) mainly from observations
of deuterium abundance as well as $^4$He abundance.
