An active material for use in a solid-state battery, comprising a core particle and a coating layer on a surface of the core particle, the core particle containing a lithium-metal complex oxide, the lithium-metal complex oxide comprising: a spinel-type complex oxide that contains Li, Mn, and O, and one or more elements other than Li, Mn, and O; or a lithium-nickel metal complex oxide that contains Li, an element M where M includes one or more elements selected from the group consisting at least of Ni, Co, Mn, and Al, and O and that has a layered structure, the coating layer comprising a compound containing Li, Nb and O, and Li being contained in the coating layer in excess of 1 mol per mole of Nb, and the active material exhibiting at least one peak in a range of 0.145 to 0.185 nm and at least one peak in a range of 0.280 to 0.310 nm in a radial distribution function obtained through measurement of an X-ray absorption fine structure thereof, wherein, in a particle size distribution, by volume, of the active material obtained through a particle size distribution measurement by laser diffraction scattering method, a ratio of an absolute value of a difference between a mode diameter of the active material and a D 10 of the active material to the mode diameter in percentage terms, (|mode diameter - D 10 | / mode diameter) × 100, satisfies 0% < ((|mode diameter - D 10 | / mode diameter) × 100) ≤ 58.0%, and wherein the active material has a crystallite size of 80 to 490 nm, and a ratio of the crystallite size to an average primary particle size determined from an image obtained using a scanning electron microscope, crystallite size / average primary particle size, is from 0.01 to 0.50. the core particle containing a lithium-metal complex oxide, the lithium-metal complex oxide comprising: a spinel-type complex oxide that contains Li, Mn, and O, and one or more elements other than Li, Mn, and O; or a lithium-nickel metal complex oxide that contains Li, an element M where M includes one or more elements selected from the group consisting at least of Ni, Co, Mn, and Al, a spinel-type complex oxide that contains Li, Mn, and O, and one or more elements other than Li, Mn, and O; or a lithium-nickel metal complex oxide that contains Li, an element M where M includes one or more elements selected from the group consisting at least of Ni, Co, Mn, and Al, and O and that has a layered structure, the coating layer comprising a compound containing Li, Nb and O, and Li being contained in the coating layer in excess of 1 mol per mole of Nb, and the active material exhibiting at least one peak in a range of 0.145 to 0.185 nm and at least one peak in a range of 0.280 to 0.310 nm in a radial distribution function obtained through measurement of an X-ray absorption fine structure thereof, wherein, in a particle size distribution, by volume, of the active material obtained through a particle size distribution measurement by laser diffraction scattering method, a ratio of an absolute value of a difference between a mode diameter of the active material and a D 10 of the active material to the mode diameter in percentage terms, (|mode diameter - D 10 | / mode diameter) × 100, satisfies 0% < ((|mode diameter - D 10 | / mode diameter) × 100) ≤ 58.0%, and wherein the active material has a crystallite size of 80 to 490 nm, and a ratio of the crystallite size to an average primary particle size determined from an image obtained using a scanning electron microscope, crystallite size / average primary particle size, is from 0.01 to 0.50.