Cognitive radio (CR) has been studied as a useful solution for efficient utilization of scarce radio spectrums. For it to succeed, two conflicting challenges are imposed on the secondary users: one is to ensure the quality of service (QoS) of the primary link, and the other is to maximize their own transmit throughput. To balance this tradeoff, beamforming and power control employing the multi-antenna in the base station (BS) of the CR network have been introduced. In the perfect beamforming situation, the power control algorithms suitable in the CR network with a multi-antenna BS (MBS) have been proposed in previous works. However, those algorithms are meaningless for realizing a practical CR network with the MBS since perfect beamforming is impossible. Therefore, unlike previous works, this paper proposes a joint beamforming and power control algorithm as a more practical strategy for realizing a CR network with the MBS. The algorithm is proposed so as to maximize the sum-rate of secondary users, while not degrading QoS for the primary link. Numerical results verify its effectiveness in a CR network with the MBS.