An all-solid-state capacitor, comprising: an inorganic solid electrolyte composed of a sintered body of a composite oxide which includes La 2/3-x Li 3x TiO 3, wherein 3x = 0.42, and having a polycrystalline structure, the polycrystalline structure being composed of crystal particles and crystal grain boundaries formed between the crystal particles; and a pair of current collectors disposed so that the inorganic solid electrolyte is interposed therebetween, the crystal particles including a first crystal particle having ionic conductivity, the first crystal particle being composed of a plurality of single crystal regions having different crystal orientations from one another, a single crystal region of the plurality of single crystal regions that is located in the outer circumferential portion of the crystal particles being larger in size than a single crystal region of the plurality of single crystal regions that is located near the center of the crystal particle, wherein the size of the single crystal region basically refers to the size of the single crystal region in a major axis direction but in case the single crystal region is adjacent to the crystal grain boundaries and the shape thereof has an aspect ratio of 3 or more, or in case the single crystal region surrounds the outer circumference of the first crystal particle, the thickness from the crystal grain boundaries to the boundaries between the single crystal region and other single crystal regions is considered as the size of the single crystal region, wherein the sintered body further includes at least one element selected from an element group consisting of Na, K, Mg, Ca, Sr and Ba, and wherein a content of the at least one element selected from the element group included in the sintered body is 1 to 9% by mass in total in terms of oxides. an inorganic solid electrolyte composed of a sintered body of a composite oxide which includes La 2/3-x Li 3x TiO 3, wherein 3x = 0.42, and having a polycrystalline structure, the polycrystalline structure being composed of crystal particles and crystal grain boundaries formed between the crystal particles; and a pair of current collectors disposed so that the inorganic solid electrolyte is interposed therebetween, the crystal particles including a first crystal particle having ionic conductivity, the first crystal particle being composed of a plurality of single crystal regions having different crystal orientations from one another, a single crystal region of the plurality of single crystal regions that is located in the outer circumferential portion of the crystal particles being larger in size than a single crystal region of the plurality of single crystal regions that is located near the center of the crystal particle, wherein the size of the single crystal region basically refers to the size of the single crystal region in a major axis direction but in case the single crystal region is adjacent to the crystal grain boundaries and the shape thereof has an aspect ratio of 3 or more, or in case the single crystal region surrounds the outer circumference of the first crystal particle, the thickness from the crystal grain boundaries to the boundaries between the single crystal region and other single crystal regions is considered as the size of the single crystal region, wherein the sintered body further includes at least one element selected from an element group consisting of Na, K, Mg, Ca, Sr and Ba, and wherein a content of the at least one element selected from the element group included in the sintered body is 1 to 9% by mass in total in terms of oxides.