A composite electrolyte comprising: an inorganic solid particle mixture comprising first inorganic solid particles having a first Li-ion conductivity at 25°C and second inorganic solid particles having a second Li-ion conductivity at 25°C, and the second Li-ion conductivity at 25°C being lower than the first Li-ion conductivity at 25°C; and a nonaqueous electrolyte comprising Li, wherein: a ratio of a weight of the nonaqueous electrolyte to a weight of the composite electrolyte is from 0.1% to 25%; the inorganic solid particle mixture satisfies 10 ≤ d90/d10 ≤ 500; wherein: d10 is a particle size [μm] at which a cumulative frequency from a small particle size side is 10% in a particle size cumulative curve of the inorganic solid particle mixture, the particle size cumulative curve being obtained by a laser diffraction scattering method; and d90 is a particle size [μm] at which a cumulative frequency from the small particle size side is 90% in the particle size cumulative curve; wherein the first inorganic solid particles are at least one selected from the group consisting of: - inorganic solid particles having a garnet-type structure; - particles of a perovskite-type Li-ion conductive material having a composition represented by a formula of La 0.56 Li 0.33 TiO 3; and - particles of a glass ceramic electrolyte having a NASICON-type structure and represented by a structural formula typified by Li 1+x+y Ti 2-x Al x P 3-y Si y O 12, where x = 0.3 and y = 0.2; and the second inorganic solid particles are at least one selected from the group consisting of: - aluminum oxide particles; - zirconium oxide particles; - silicon oxide particles; - magnesium oxide particles; - rutile type titanium oxide particles; - niobium oxide particles; - tantalum oxide particles; - hafnium oxide particles; - yttrium oxide particles; - gallium oxide particles; - germanium oxide particles; and - lanthanoid oxide particles. an inorganic solid particle mixture comprising first inorganic solid particles having a first Li-ion conductivity at 25°C and second inorganic solid particles having a second Li-ion conductivity at 25°C, and the second Li-ion conductivity at 25°C being lower than the first Li-ion conductivity at 25°C; and a nonaqueous electrolyte comprising Li, wherein: a ratio of a weight of the nonaqueous electrolyte to a weight of the composite electrolyte is from 0.1% to 25%; the inorganic solid particle mixture satisfies 10 ≤ d90/d10 ≤ 500; wherein: d10 is a particle size [μm] at which a cumulative frequency from a small particle size side is 10% in a particle size cumulative curve of the inorganic solid particle mixture, the particle size cumulative curve being obtained by a laser diffraction scattering method; and d90 is a particle size [μm] at which a cumulative frequency from the small particle size side is 90% in the particle size cumulative curve; a ratio of a weight of the nonaqueous electrolyte to a weight of the composite electrolyte is from 0.1% to 25%; the inorganic solid particle mixture satisfies 10 ≤ d90/d10 ≤ 500; wherein: d10 is a particle size [μm] at which a cumulative frequency from a small particle size side is 10% in a particle size cumulative curve of the inorganic solid particle mixture, the particle size cumulative curve being obtained by a laser diffraction scattering method; and d90 is a particle size [μm] at which a cumulative frequency from the small particle size side is 90% in the particle size cumulative curve; d10 is a particle size [μm] at which a cumulative frequency from a small particle size side is 10% in a particle size cumulative curve of the inorganic solid particle mixture, the particle size cumulative curve being obtained by a laser diffraction scattering method; and d90 is a particle size [μm] at which a cumulative frequency from the small particle size side is 90% in the particle size cumulative curve; wherein the first inorganic solid particles are at least one selected from the group consisting of: - inorganic solid particles having a garnet-type structure; - particles of a perovskite-type Li-ion conductive material having a composition represented by a formula of La 0.56 Li 0.33 TiO 3; and - particles of a glass ceramic electrolyte having a NASICON-type structure and represented by a structural formula typified by Li 1+x+y Ti 2-x Al x P 3-y Si y O 12, where x = 0.3 and y = 0.2; and - inorganic solid particles having a garnet-type structure; - particles of a perovskite-type Li-ion conductive material having a composition represented by a formula of La 0.56 Li 0.33 TiO 3; and - particles of a glass ceramic electrolyte having a NASICON-type structure and represented by a structural formula typified by Li 1+x+y Ti 2-x Al x P 3-y Si y O 12, where x = 0.3 and y = 0.2; and the second inorganic solid particles are at least one selected from the group consisting of: - aluminum oxide particles; - zirconium oxide particles; - silicon oxide particles; - magnesium oxide particles; - rutile type titanium oxide particles; - niobium oxide particles; - tantalum oxide particles; - hafnium oxide particles; - yttrium oxide particles; - gallium oxide particles; - germanium oxide particles; and - lanthanoid oxide particles. - aluminum oxide particles; - zirconium oxide particles; - silicon oxide particles; - magnesium oxide particles; - rutile type titanium oxide particles; - niobium oxide particles; - tantalum oxide particles; - hafnium oxide particles; - yttrium oxide particles; - gallium oxide particles; - germanium oxide particles; and - lanthanoid oxide particles.