A frame transmission prevention apparatus connected to a network of a network system including a plurality of electronic control units communicating with one another via the network is provided. The apparatus includes a processor and a memory. The memory includes at least one set of instructions that causes the processor to perform processes when executed by the processor. The processes include receiving a first frame from the network and switching whether to perform a first process for preventing transmission of the first frame on the basis of management information indicating whether prevention of transmission of a frame is permitted if the first frame satisfies a first condition.