A polyion complex of mRNA and a polycationic polymer, and a composition and a pharmaceutical composition for mRNA delivery are provided. The polyion complex of mRNA and the polycationic polymer can deliver mRNA into cells in the body of a subject almost without inducing inflammation. The polyion complex can then cause mRNA to be uniformly expressed in cells in the body of the subject. The pharmaceutical composition containing the polyion complex is suitable for use in treating a disease whose condition rapidly progresses or an acute disease.