  We have studied the rationally frustrated Josephson-junction array model in
the square lattice through Monte Carlo simulations of $(2+1)$D XY-model. For
frustration $f=1/4$, the model at zero temperature shows a continuous
superfluid-insulator transition. From the measurement of the correlation
function and the superfluid stiffness, we obtain the dynamical critical
exponent $z=1.0$ and the correlation length critical exponent $\nu=0.4 \pm
0.05$. While the dynamical critical exponent is the same as that for cases
$f=0$, 1/2, and 1/3, the correlation length critical exponent is surprisingly
quite different. When $f=1/5$, we have the nature of a first-order transition.
