In a cellular communication system represented by, for example, 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), a mobile station (hereinafter “UE” (user equipment)) is required to perform measurement process in order to perform mobility control such as a handover.  This measurement includes intra-frequency measurement, inter-frequency measurement using gaps and inter-system measurement, and a cellular communication system is required to support these measurements.  Also, inter-frequency measurement using gaps and inter-system measurement are also referred to as gap-assisted measurement. 
To perform gap-assisted measurement, UE is required to receive a signal from another cell with different carrier frequency or from another system, so that UE has to adjust its receiver away from the frequency of the source cell to another carrier frequency or another system of a neighbor cell.  For UE to measure a neighbor cell, it is necessary to provide gaps (hereinafter also referred to as “an idle period”) to UE. 
In order to achieve gap synchronization between a serving base station and UE, explicit start position to activate gaps is configured.  Also, gaps are arranged on a periodic basis, and these periodic gaps are referred to as a gap pattern.  To perform measurement, this gap pattern needs to be provided over a long period.  Therefore, by performing gap-assisted measurement base on the gap pattern arrangement, UE can support mobility control to other carrier frequencies or other systems even during communication.  In addition, even during discontinuous reception (DRX), measurement is possible only after gap is activated. 
In addition to the above measurement process, gaps are used to receive broadcast information (also referred to as “system information”) of a specific cell.  Specifically, gaps are also used to identify whether or not it is possible to access the cell called closed subscriber group (CSG) cell where only specific UE can access, by comparing the CSG identifier of this cell that is included in broadcast information and an accessible CSG identifier list that UE has.  Since UE cannot receive broadcast information from other cells during communication with a serving base station, UE acquires broadcast information of other cells using gaps. 
Meanwhile, by explicit signaling is used to indicate the start of a gap pattern, a delay is expected until UE starts measurement.  This is because there are a delay for decision to generate gaps in a base station, and a delay for signaling transmission to indicate the start of a gap pattern. 
Therefore, conventionally, the method to measure using DRX without designating gaps by explicit signaling, and the method to start measurement based on a CQI value measured without designating gaps by explicit signaling disclosed in non-patent literature 1 have been considered.  In the former method, it is not necessary to provide an explicit gap pattern, so that it enables UE to start measurement earlier. 
In the latter method, as disclosed in non-patent literature 1, if the CQI value is lower than the defined threshold or the configured threshold, UE voluntary starts measurement step of using gap.  A base station receives CQI reporting that implicitly shows that UE starts measurement and by this means can detect that UE starts measurement.