We study the equation of state in 2+1 flavor QCD with nonperturbatively improved Wilson quarks coupled with the renormalization group-improved Iwasaki glue. We apply the T-integration method to nonperturbatively calculate the equation of state by the fixed-scale approach. With the fixed-scale approach, we can purely vary the temperature on a line of constant physics without changing the system size and renormalization constants. Unlike the conventional fixed-N t approach, it is easy to keep scaling violations small at low temperature in the fixed-scale approach. We study 2+1 flavor QCD at light quark mass corresponding to m π/m ρ≲0.63, while the strange quark mass is chosen around the physical point. Although the light quark masses are still heavier than the physical values, our equation of state is roughly consistent with recent results with highly improved staggered quarks at large N t. © 2012 American Physical Society.