For a variant of RSA with modulus N = p^rq and ed ≡ 1 (mod(p-1)(q-1)), we show that d is to be recovered if d < N^{(2-√2)/(r+1)}. (Note that φ(N) ≠ (p-1)(q-1).) Boneh-Durfee's result for the standard RSA is obtained as a special case for r=1. Technically, we develop a method for finding a small root of a trivariate polynomial equation f(x, y, z)=x(y-1)(z-1)+1 ≡ 0 (mod e) under the condition that y^rz=N. Our result cannot be obtained from the generic method of Jochemsz-May.