  For a positive integer $N$ divisible by $4,5,6,7$ or $9$, let
$\mathcal{O}_{1,N}(\mathbb{Q})$ be the ring of weakly holomorphic modular
functions for the congruence subgroup $\Gamma_1(N)$ with rational Fourier
coefficients. We present explicit generators of the ring
$\mathcal{O}_{1,N}(\mathbb{Q})$ over $\mathbb{Q}$ by making use of modular
units which have infinite product expansions.
