The film was nearly banned in Japan due to the fact that the makeup that some of the cast wore as they were turning into mushrooms was very reminiscent of how many people looked after the atomic bombing of Hiroshima and Nagasaki.
The giant mushrooms that the characters eat were created by the special effects crew from rice pastry. The rice pastry would be picked up from a nearby shop that specialized in it, and taken to the studio where it was shaped and cooked. Because Kumi Mizuno was so well liked, they would add sugar and other flavors.
The effect of the growing mushrooms was done through the same means later used to create the "instant pudding" in Woody Allen's Sleeper(1973).
Based on the story "Voice in the Night" by William Hope Hodgson, which was also adapted for the 24th episode of the 1958 anthology series Suspicion.