A lithium-titanium complex oxide whose total water generation amount and total carbon dioxide generation amount measured by thermal decomposition GC-MS are preferably 1500 wt ppm or less and 2000 wt ppm or less, respectively, is obtained by subjecting a mixture of titanium compound and lithium compound to a heat treatment at 600° C. or above, cooling the obtained reaction product to 50° C. or below, and then subjecting the cooled reaction product to a reheat treatment involving heating to the maximum temperature of 300 to 700° C. and then cooling, wherein the dew point of the ambience of the reheat treatment is controlled at −30° C. or below at a temperature of 200° C. or above.