The invention relates to a lithium lanthanum titanate composite solid electrolyte material containing silicon in which amorphous Si or an amorphous Si compound exist in a grain boundary between crystal grains, and a method of producing the same, and belongs to a field of a lithium ion battery. According to the invention, the amorphous Si or the amorphous Si compound exist in the grain boundary between the crystal grains of the lithium lanthanum titanate. The amorphous Si or the amorphous Si compound are introduced into the grain boundary by employing a wet chemical method. In the wet chemical method, the inexpensive organosilicon compound is used as an additive, and the organosilicon compound is added into the lithium lanthanum titanate solid electrolyte material. Thus, it is possible to synthesize the lithium lanthanum titanate composite solid electrolyte material containing silicon by performing sintering when the ratio of mass of the Si or mass of the Si calculated based on mass of the Si compound to mass of the lithium lanthanum titanate is 0.27% to 1.35%.