  Let $\mathbb{F}_{q}$ be a finite field with $q$ elements and
$\mathbb{F}_{q}[x]$ the ring of polynomials over $\mathbb{F}_{q}$. Let $l(x),
k(x)$ be coprime polynomials in $\mathbb{F}_{q}[x]$ and $\Phi(k)$ the Euler
function in $\mathbb{F}_{q}[x]$. Let $\pi(l, k; n)$ be the number of monic
irreducible polynomials of degree $n$ in $\mathbb{F}_{q}[x]$ which are
congruent to $l(x)$ module $k(x)$. For any positive integer $n$, we denote by
$\Omega(n)$ the least prime divisor of $n$. In this paper, we show that
$$\pi(l, k;
n)=\frac{1}{\Phi(k)}\frac{q^{n}}{n}+O\left(n^{\alpha}\right)+O\left(\frac{q^{\frac{n}{\Omega{(n)}}}}{n}\right),$$
where $\alpha$ only depends on the choice of $k(x)\in\Fq$. Note that the above
error term improves the one implied by Weil's conjecture. Our approach is
completely elementary.
