From Wikizilla

The Showa era is a term used to identify the years between 1930 and 1979.

The Showa Era to Godzilla

The first Godzilla film was to begin the Showa era of the daikaiju industry, and is the usual kaiju to be affiliated with this eiga. During the beginning of this eiga, Godzilla was mainly antagonistic in nature, taking on the likes of Anguirus and Mothra. By the time of Ghidorah, the Three-Headed Monster, Godzilla adopted what would soon become his most famous image: a heroic monster, battling terrible villains with other monsters by his side. In fact, many of the monsters affiliated with Godzilla (Rodan, Ghidorah, Gigan, Megalon, etc.) appeared during this eiga.