Laser-accelerated ions are widely attractive for many fields as a compact and low-cost accelerator. For the application to medical treatment, it is necessary to not only enhance the maximum acceleration energy but also improve beam stability and quality. In this study, we demonstrate a new ion acceleration mechanism using a uniform critical density plasma that yields very high ion energy despite the existence of a laser prepulse. The maximum proton energy in the experiment was approximately 18 MeV accelerated by the laser pulse of 3 x 1019 W/cm2 focused intensity under the conditions of the maximum prepulse contrast ratio of 10-3. Further, heavier particles such as carbon or oxygen present in the plasma were accelerated using the same acceleration field. In addition, a self-created magnetic field in the plasma significantly improved emission divergence.