A method for manufacturing a transparent complex oxide sintered body comprising the steps of: (i) preparing a source powder based on a rare earth oxide represented by the compositional formula: (Tb x Y 1-x) 2 O 3 wherein x is a number from 0.4 to 0.6 and a sintering aid comprising oxide of at least one element selected from Group 2 elements and Group 4 elements, wherein the source powder consists essentially of said rare earth oxide and the sintering aid, and the total of the rare earth oxide is at least 95% by weight of the source powder, the source powder being prepared by - furnishing an aqueous solution containing (a) terbium ions, (b) yttrium ions, and (c) ions of said at least one element of the sintering aid; - letting components (a), (b) and (c) co-precipitate from the solution; - filtering the co-precipitate; - drying the co-precipitate by heating at 100°C or less and/or firing it by heating at 500 to 1,000°C in an oxygen-containing atmosphere; - heat treating the resulting co-precipitate or oxide thereof, the heat treatment including heating in an oxygen-containing atmosphere at a temperature of from 800°C to less than 1,200°C and then cooling in an inert atmosphere, vacuum or reducing atmosphere down to 350°C or below; (ii) molding a compact from the source powder; (iii) subjecting the molded compact to binder burnout at a temperature of not more than 350°C, whereby the level of trivalent terbium in the compact is such that a powder sample taken from the compact after the binder burnout, and heated in air from room temperature at a heating rate of 15°C/min, exhibits a weight gain due to oxidative reaction in a range of from 250°C to 500°C of at least y%, y being determined by the formula: y = 2 x + 0.3 where x is as in the above compositional formula; (iv) sintering the compact in an inert atmosphere or vacuum; (v) hot isostatic pressing the sintered compact to form a transparent complex oxide sintered body which, when excited with excitation light of wavelength 380 nm, emits light including a component of wavelength 545 nm which has a luminescent lifetime of at least 0.3 ms, and wherein a specimen 11 mm long made from the sintered body has an overall light transmittance of at least 81.0% at a wavelength of 1,064 nm. (i) preparing a source powder based on a rare earth oxide represented by the compositional formula: (Tb x Y 1-x) 2 O 3 wherein x is a number from 0.4 to 0.6 and a sintering aid comprising oxide of at least one element selected from Group 2 elements and Group 4 elements, wherein the source powder consists essentially of said rare earth oxide and the sintering aid, and the total of the rare earth oxide is at least 95% by weight of the source powder, the source powder being prepared by - furnishing an aqueous solution containing (a) terbium ions, (b) yttrium ions, and (c) ions of said at least one element of the sintering aid; - letting components (a), (b) and (c) co-precipitate from the solution; - filtering the co-precipitate; - drying the co-precipitate by heating at 100°C or less and/or firing it by heating at 500 to 1,000°C in an oxygen-containing atmosphere; - heat treating the resulting co-precipitate or oxide thereof, the heat treatment including heating in an oxygen-containing atmosphere at a temperature of from 800°C to less than 1,200°C and then cooling in an inert atmosphere, vacuum or reducing atmosphere down to 350°C or below; - furnishing an aqueous solution containing (a) terbium ions, (b) yttrium ions, and (c) ions of said at least one element of the sintering aid; - letting components (a), (b) and (c) co-precipitate from the solution; - filtering the co-precipitate; - drying the co-precipitate by heating at 100°C or less and/or firing it by heating at 500 to 1,000°C in an oxygen-containing atmosphere; - heat treating the resulting co-precipitate or oxide thereof, the heat treatment including heating in an oxygen-containing atmosphere at a temperature of from 800°C to less than 1,200°C and then cooling in an inert atmosphere, vacuum or reducing atmosphere down to 350°C or below; (ii) molding a compact from the source powder; (iii) subjecting the molded compact to binder burnout at a temperature of not more than 350°C, whereby the level of trivalent terbium in the compact is such that a powder sample taken from the compact after the binder burnout, and heated in air from room temperature at a heating rate of 15°C/min, exhibits a weight gain due to oxidative reaction in a range of from 250°C to 500°C of at least y%, y being determined by the formula: y = 2 x + 0.3 where x is as in the above compositional formula; whereby the level of trivalent terbium in the compact is such that a powder sample taken from the compact after the binder burnout, and heated in air from room temperature at a heating rate of 15°C/min, exhibits a weight gain due to oxidative reaction in a range of from 250°C to 500°C of at least y%, y being determined by the formula: y = 2 x + 0.3 where x is as in the above compositional formula; (iv) sintering the compact in an inert atmosphere or vacuum; (v) hot isostatic pressing the sintered compact to form a transparent complex oxide sintered body which, when excited with excitation light of wavelength 380 nm, emits light including a component of wavelength 545 nm which has a luminescent lifetime of at least 0.3 ms, and wherein a specimen 11 mm long made from the sintered body has an overall light transmittance of at least 81.0% at a wavelength of 1,064 nm.