From a series of plastic model kits produced by Imai, called Megaro Zamac. There was never an anime show related to this series, I think Imai simply produced the kits to cash in on the giant robot model craze. This is one of the hero mecha, the Jha-Giga, I stuck fairly closely to the original design but added a lot of extra detail since the models were fairly primitive compared to more modern anime mecha kits.
