A method performed by a terminal in a wireless communication system, the method comprising: transmitting, to a base station, a request for system information, SI; in case that an acknowledgement for the request is received from the base station: monitoring a physical downlink control channel, PDCCH, addressed to an SI radio network temporary identifier, RNTI, from a start of a first SI window, after receiving the acknowledgement; and receiving an SI message based on the PDCCH, wherein the monitoring is performed until an end of the first SI window or until the SI message is received, an absolute length in time of the first SI window being configured by the base station, and wherein the monitoring is performed in a second SI window which is next to the first SI window in a modification period to which the first SI window belongs, in case that the SI message is not received by the end of the first SI window; and in case that a cell reselection occurs while the terminal is waiting for the acknowledgement for the request: resetting a medium access control, MAC; and releasing a radio link control, RLC, entity for a signaling radio bearer 0, SRBO, in case that the request is based on an RRC system information request message for requesting the system information. transmitting, to a base station, a request for system information, SI; in case that an acknowledgement for the request is received from the base station: monitoring a physical downlink control channel, PDCCH, addressed to an SI radio network temporary identifier, RNTI, from a start of a first SI window, after receiving the acknowledgement; and receiving an SI message based on the PDCCH, wherein the monitoring is performed until an end of the first SI window or until the SI message is received, an absolute length in time of the first SI window being configured by the base station, and wherein the monitoring is performed in a second SI window which is next to the first SI window in a modification period to which the first SI window belongs, in case that the SI message is not received by the end of the first SI window; and monitoring a physical downlink control channel, PDCCH, addressed to an SI radio network temporary identifier, RNTI, from a start of a first SI window, after receiving the acknowledgement; and receiving an SI message based on the PDCCH, wherein the monitoring is performed until an end of the first SI window or until the SI message is received, an absolute length in time of the first SI window being configured by the base station, and wherein the monitoring is performed in a second SI window which is next to the first SI window in a modification period to which the first SI window belongs, in case that the SI message is not received by the end of the first SI window; and in case that a cell reselection occurs while the terminal is waiting for the acknowledgement for the request: resetting a medium access control, MAC; and releasing a radio link control, RLC, entity for a signaling radio bearer 0, SRBO, in case that the request is based on an RRC system information request message for requesting the system information. resetting a medium access control, MAC; and releasing a radio link control, RLC, entity for a signaling radio bearer 0, SRBO, in case that the request is based on an RRC system information request message for requesting the system information.