Volterra operator, continuity on Sobolev space

Question

Given an integral operator the form: $T \colon D(\mathbb{R}) \to D'(\mathbb{R})$ mapping $f \mapsto \int_{-\infty}^{y} f(x)\,dx$ I want to prove that it extends by continuity to Sobolev spaces of order less then $0$. So I thought a way is to show that $T$ is a pseudodifferential operator of order $< 0$. What is the easiest way to check this?