We prepared a new series of rare earth photocatalysts, Bi2RNbO7 (R = Y, rare earth), and found that the variation of R ion radius r(R)(3+) in Bi2RNbO7 led to a change in the band structure and the activity of photocatalytic decomposition of water. The band gap E-g increases with increasing r(R)(3+) while the photocatalytic activity A(R)(3+) decreases with increasing r(R)(3+). The analysis of the E(r(R)(3+)) dependency showed that the position of the 4f band of the R ion in Bi2RNbO7 determines the band gap E-g, resulting in the difference in photocatalytic activity.