Industrial Internet of Things (IIoT) is a significant component of the future industrial systems. Cyber security is the top concern for IIoT adoption. In this paper, we surveyed the recent research work around IIoT security. Since applications for the IIoT is a natural evolution of the IoT, IIoT inherits some security challenges from IoT due to the similar basic architecture. Meanwhile, differences still exist between IoT and IIoT, and hence IIoT has some specific security concerns mainly on protecting critical industry control systems. Thus, we categorized the security challenges in IIoT systems into two groups: the security challenges that exist in both IoT and IIoT, and the security challenges specific to Industry systems. In each group, we analyzed the specific security challenges and studied the previous work on addressing these challenges.