An electrolysis apparatus is disclosed as including an electrolysis cell (10) accommodating therein electrolyte (70), a heating section (20) located around the electrolysis cell to heat the electrolysis cell, an electrode section (30) having an electrode unit (30a) immersed in the electrolyte and a power-conducting electrode portion (30b) supporting the electrode unit to apply the electrode unit with electric power, a lid body (45) defining a space region (40) in an area above the electrolysis cell, an exhaust section (50) located in the lid body to allow the space region to communicate with an outside for exhausting by-product gas, resulting from electrolysis of the electrolyte, from the space region to the outside, and an evaporation restraining member (60, 80, 80A, 80B, 90, 90A, 90B) floating on a liquid surface of the electrolyte so as to cover the liquid surface of the electrolyte for permitting by-product gas, resulting from electrolysis of the electrolyte, to escape to the space region while restraining the electrolyte from evaporating.