A radio communications device includes a first error detection part configured to perform error detection on a header included in a packet; a determination part configured to determine whether there is consistency with respect to the length of the packet based on the header in response to the first error detection part detecting no error in the header; a decryption part configured to decrypt the packet in response to the determination part determining that there is consistency with respect to the length of the packet; and a second error detection part configured to perform error detection on the packet in response to the determination part determining that there is consistency with respect to the length of the packet, wherein the decryption part is configured to start to decrypt the packet before completion of the error detection by the second error detection part.