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 $Φ(k)$ the Euler function in $\mathbb{F}_{q}[x]$. Let $π(l, k; n)$ 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 $Ω(n)$ the least prime divisor of $n$. In this paper, we show that $$π(l, k; n)=\frac{1}{Φ(k)}\frac{q^{n}}{n}+O\left(n^α\right)+O\left(\frac{q^{\frac{n}{Ω{(n)}}}}{n}\right),$$ where $α$ 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.