To provide a transmission right acquisition method which reduces a deterioration of a communication quality and enables more simultaneous communications.

A controller 60 sets a value of CW to CW_MIN. The controller 60 initializes a value of a reset counter. The controller 60 confirms whether or not the value of the reset counter has become "0". When is it not "0", a back-off time determining portion 50 selects a back-off time from among a section of [(1/2)*CW, CW] at random. According to the selected back-off time, a timer 54 counts a standby time. A radio transmitting/receiving portion 42 starts a transmission of a voice packet. The radio transmitting/receiving portion 42 confirms the presence/absence of a collision of the voice packet transmission. When the collision of the voice packet transmission is present, the radio transmitting/receiving portion 42 stops the transmission of the voice packet. A competitive window setting portion 52 updates a value of CW. A smaller one of a double value of the present CW value and CW_MAX is selected as an updated value.

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