An oxygen supply controlling apparatus of boiler having wind boxes (2a,2b) disposed on the right and left of a boiler main body (1). In the control of the rate of oxygen supplied, the apparatus is designed to regulate the degree of opening for oxygen supply by means of right and left oxygen supply regulators (15a,15b) on the basis of oxygen supply flow rate and right and left actually measured oxygen concentrations, thereby attaining control of oxygen supply balance. Further, in the control of exhaust gas flow rate, the apparatus is designed to correct the preset value for exhaust gas flow rate corresponding to boiler load instructions on the basis of the preset value for oxygen concentration corresponding to boiler load instructions, right and left actually measured exhaust gas flow rates and right and left actually measured oxygen concentrations and accordingly regulate the degree of opening for exhaust gas introduction by means of right and left exhaust gas introduction regulators (8a,8b), thereby attaining control of exhaust gas flow rate.