A robot controller having a function for controlling the motion of a robot, comprising a processor for controlling the robot and for processing the environmental condition data representing the environmental conditions for the robot, a memory accessible by the processor, a writing unit for executing a function of writing the environmental condition into the memory, a first data bus connected to the memory, and a second data bus having a transfer rate lower than that of the first data bus and for transferring the control data used for controlling the robot, wherein the writing unit writes the environmental condition data into the memory through the first data bus, not through the second data bus. The writing unit can be a processor control chip which is connected to the processor through the first data bus.