A wireless base station for transmitting a data packet to a plurality of mobile stations through a downlink shared channel determines two powers to be allocated to the downlink shared channel using two large and small transmission power margins. The wireless base station carries out a step of relating a first transmission format, which applies a phase modulation or a phase-amplitude modulation to a data packet to be transmitted by an allocation power corresponding to the larger margin, to the mobile stations and a step of relating a second transmission format, which applies only the phase modulation to the data packet to be transmitted using the allocation power corresponding to the other margin, to the mobile stations, compares the total numbers of bits to be transmitted by the respective transmission formats, and transmits the data packet by the transmission format corresponding to a larger value. With this operation, the communication fault of an HSDPA due to the transmission power margins can be prevented.