The present invention is directed to preventing occurrence of PDCCH blocking while reducing PDCCH detection errors. The present invention provides a search space size adaptive control section (203) that controls the size of the UE-specific search space adaptively depending on the number of component carriers to which a PDSCH for a mobile station apparatus (10) is assigned, a control information generation section (201) that generates control information reflecting a control result by the search space size adaptive control section (203), and a transmission/reception section (23) that transmits this control information to a mobile station apparatus (10) by RRC signaling.