NTT DoCoMo Inc. reported a 21 percent drop in operating profit for the April-September period Friday, reflecting its loss of customers to rivals KDDI Corp. and Softbank Corp.

Group operating profit at DoCoMo totaled ¥408.5 billion for the six months to Sept. 30, down from ¥516.9 billion a year earlier, due mainly to investments in increasing the number of its base stations and rising advertisement costs.

Net profit fell 20.4 percent to ¥246.5 billion from ¥309.8 billion. Sales fell 2.4 percent to ¥2.32 trillion from ¥2.38 trillion a year ago.