normal mode in accordance with the second mode value positive stored in the buffer memory 124 i e a value that instructs the terminal to be ready since there are data to be transmitted steps s 216 and s 218 that is if the base station 110 has data to be transmitted to the terminal 100 the traffic indication message has the second mode value positive at this time if mode switching is completed the buffer memory 124 preferably deletes the second mode value temporarily stored in step s 214 if the portable internet unit 120 is switched to the normal mode in step s 218 the terminal 100 transmits a data transmission request message to the base station 110 through the portable internet unit 120 to notify that the portable internet unit 120 is currently in the normal mode step s 220 accordingly the base station 110 transmits certain data to be transmitted to the portable internet unit 120 of the terminal 100 step s 222 next although not shown in the drawings a case where an embedded controller or a microcomputer is employed as the mode control unit 130 will be briefly described if a communication service is not used the terminal 100 transmits a sleep request message mob_slp req to the base station 110 through the rf transceiver unit 122 in order to switch the portable internet unit 120 of the terminal 100 to the sleep mode then the base station 110 determines whether to approve the transmitted sleep request message mob_slp req and transmits a sleep response message mob_slp rsp to the terminal 110 in order to switch the portable internet unit 120 to the sleep mode in response thereto the terminal 100 switches the portable internet unit 120 to the sleep mode next.