This paper designs and implements an IIoT(Industrial Internet of Things) platform based on MMS(Manufacturing Message Specification) with enhanced security through unidirectional data transmission. Unidirectional gateways that are used to overcome transferring between separated networks, are capable of transferring information between networks, but require a one-to-one connection between networks, and additional gateways are required for additional network connections. In addition, there is a disadvantage that data reliability is low due to unidirectional communication through UDP. MMS is an industrial message protocol that guarantees stability and real-time. In order to apply the MMS to the IoT environment, one-to-many transmission of messages must be possible and support in a private network should be possible. Therefore, IoT communication protocols are investigated and analyzed to use MMS in industrial IoT environment. Based on the analysis results, the proposed platform supports unidirectional communication between networks using MQTT (Message Queue Telemetry Transport) broker, which acts as a gateway used in unidirectional communication. At this time, unidirectional communication restricts data transmission from a network with low security level to a network with high security level based on a designed unidirectional data transmission protocol. In addition, IIMP (Industrial IoT Message Protocol) supporting MMS in IIoT environment is designed using publishing and subscription functions of MQTT. In order to evaluate the performance of the proposed platform, performance of UDP that used in the existing unidirectional gateway, is compared. Simulation results show that the proposed platform has a slower transmission time then UDP, but the reliability of data transmission is higher when a large number of packets are transmitted to the one port.