In the mobile radio communication system where the UMTS and an another system coexist an idle slot for observing the frequency component of another system is inserted in one superframe of UMTS. The duration of this idle slot is at most half of the duration of one frame that form the superframe and it is inserted at an interval of a specified number of frames. Therefore, the frequency component of the another system can be observed securely from the UMTS. Further, deterioration of interleaving performance of the compressed mode frames during such observation can be suppressed.