A design assistance device assists design of a processing device to execute processing and includes an acquirer, divider, communication code generator, and outputter. The acquirer acquires processing code describing content of the processing. The divider divides the processing code into modules and determines, for each of the modules, an execution environment in which the module is to be executed, from among a plurality of execution environments provided on the processing device. The communication code generator generates, when a module of the plurality of modules includes partial processing prioritized for execution in a second execution environment other than a first execution environment determined for execution of the module, a communication code for communication between the first execution environment and the second execution environment to execute the partial processing in the second execution environment. The outputter outputs the plurality of modules and the communication code to the processing device.