Controlling the pressure of oil wells during drilling can be one of the most complex and dangerous processes of the exploration stage. This study proposes the design of an improved internal model controller (IMC) to control the pressure at the bottom of wells during drilling operations based on Managed Pressure Drilling (MPD). In the first part of this work, there was obtained a mathematical linear model of the process, which is founded on fluid mechanics. The dynamic process showed an integrator element, moreover, is considered the addition of a time-delay between the action of the valve and the response of the downhole pressure variation. In the second part, the improved IMC controller was designed to offset the effect of the integrate term with time delay looking for the best performance and robustness of the system. Finally, the proposed controller is tested by common problems during drilling simulations (loss of fluid, influxes, pipe connection, and loss of pump power) showing its viability.