An explosion-proof plug includes a first casing part having two ends and an outer wall having an edge and defining a compression-resistant annular chamber being open toward one of the ends, an annular transformer disposed in the annular chamber, a cover secured to the first casing part covering the annular chamber and the transformer, a second substantially bowl-shaped casing part adjacent the edge of the outer wall, the second casing part and the cover together defining a first connection space with increased security as compared to other portions of the plug for receiving connecting cables to be connected to the transformer, a third casing part securely connected to the first casing part defining a second space of increased security, and plug pins being held in the third casing part and connected to the transformer.