A polycrystalline YAG sintered body, wherein, when dimensions of a smallest rectangular solid surrounding a YAG sintered body are A mm×B mm×C mm, a maximum value (A, B, C) is 150 mm or less, a minimum value (A, B, C) is more than 20 mm and 40 mm or less, and an optical loss coefficient when light of a wavelength of 300 to 1500 nm (excluding wavelengths which result in absorption of light by an additive element) is transmitted therethrough is 0.002 cm−1 or less. Moreover, a polycrystalline YAG sintered body, wherein, when dimensions of a smallest rectangular solid surrounding a YAG sintered body are A mm×B mm×C mm, a maximum value (A, B, C) is more than 150 mm and 300 mm or less, a minimum value (A, B, C) is more than 5 mm and 40 mm or less, and an optical loss coefficient when light of a wavelength of 300 to 1500 nm (excluding wavelengths which result in absorption of light by an additive element) is transmitted therethrough is 0.002 cm−1 or less. An object of an embodiment of the present invention is to provide a large and transparent polycrystalline YAG sintered body and its production method.