Chemical abundances in cosmic rays are crucial information about their origin and acceleration mechanisms. The observed chemical composition of the cosmic rays shows enhancements to the solar abundances. The cause of the chemical enhancements of some ions has not been understood. It is not clear, in relativistic collisionless shocks propagating in multispecies-ion plasmas, how each ion and electron is accelerated. We perform the first Particle-in-Cell simulation of a relativistic collisionless shock propagating in an electron-proton-helium plasma. We found that helium ions are accelerated efficiently compared with protons.