An electrolyte sheet for a solid oxide fuel cell and to be installed in said fuel cell, wherein at least one surface of the electrolyte sheet has 30 or less cracks in each section as detected by a fluorescent flaw-detecting penetration test, the each section being obtained by dividing the electrolyte sheet into sections such that a length of each side of the section is 30 mm or less, and the electrolyte sheet is comprising a zirconia-based oxide, wherein the zirconia-based oxide is a zirconia stabilized with an oxide of at least one element selected from the group consisting of scandium, yttrium, cerium and ytterbium and wherein the fluorescent flaw-detecting penetration test comprises the steps of: (i) impregnating a surface of the electrolyte sheet with a water-soluble fluorescent penetrant solution as a fluorescent agent for 5 minutes so as to penetrate the fluorescent agent into cracks on the surface of the electrolyte sheet, and washing the surface of the electrolyte sheet with water spray and drying the sheet; (ii) then, continuously irradiating the electrolyte sheet with a near-ultraviolet radiation of a wavelength of 375 nm using a black light for fluorescent flaw detection in a darkroom and taking a fluorescence still image of the electrolyte sheet with a CCD imaging camera; (iii) then, irradiating the electrolyte sheet with visible light for a period of 1/1000 sec with a white stroboscope and taking a visible still image for determining the outline of the electrolyte sheet with the CCD imaging camera; and (iv) then, displaying a superimposed image obtained by superimposing the fluorescence still image and the visible still image, identifying an inspection area in the visible still image by pattern-matching the visible still image with the shape of the electrolyte sheet, eliminating, from the fluorescence still image, an image outside the inspection area that is determined as each of sections obtained by dividing the electrolyte sheet, each of the sections having a side with a length of 30 mm or less, identifying fluorescent spots by morphology-processing the fluorescence still image, and calculating the number of the fluorescent spots by pattern-matching the visible still image with the shape of the electrolyte sheet, wherein the electrolyte sheet is formed from a green sheet including a binder whose acid value ratio is 0.01 to 0.3. (i) impregnating a surface of the electrolyte sheet with a water-soluble fluorescent penetrant solution as a fluorescent agent for 5 minutes so as to penetrate the fluorescent agent into cracks on the surface of the electrolyte sheet, and washing the surface of the electrolyte sheet with water spray and drying the sheet; (ii) then, continuously irradiating the electrolyte sheet with a near-ultraviolet radiation of a wavelength of 375 nm using a black light for fluorescent flaw detection in a darkroom and taking a fluorescence still image of the electrolyte sheet with a CCD imaging camera; (iii) then, irradiating the electrolyte sheet with visible light for a period of 1/1000 sec with a white stroboscope and taking a visible still image for determining the outline of the electrolyte sheet with the CCD imaging camera; and (iv) then, displaying a superimposed image obtained by superimposing the fluorescence still image and the visible still image, identifying an inspection area in the visible still image by pattern-matching the visible still image with the shape of the electrolyte sheet, eliminating, from the fluorescence still image, an image outside the inspection area that is determined as each of sections obtained by dividing the electrolyte sheet, each of the sections having a side with a length of 30 mm or less, identifying fluorescent spots by morphology-processing the fluorescence still image, and calculating the number of the fluorescent spots by pattern-matching the visible still image with the shape of the electrolyte sheet,