With popularization of cloud storage services, there appears to share outsourced data by multiple users for cooperative work with minimum storage costs. For data integrity and consistency in the cloud service, the audit mechanisms were proposed. However, existing approaches have some security vulnerabilities and require a lot of computational overheads. This paper proposes a secure and efficient audit mechanism for dynamic shared data in cloud storage. The proposed scheme prevents a malicious cloud service provider from deceiving the auditor. Moreover, it devises a new index table management method and reduces the auditing costs by employing less complex operations. We prove the resistance against some attacks and show less computational costs and shorter time for auditing to compare with conventional approaches. The results present that the proposed scheme is secure and efficient for cloud storage services managing dynamic shared data.