[Problem] To provide a low-cost and simple method for precisely detecting airtightness errors in an operating gas circulation-type gas engine, and to provide an operating gas circulation-type gas engine using said method. [Solution] In an operating gas circulation-type gas engine, a fuel gas and/or an oxidant gas and/or an operating gas is supplied in excess by a prescribed amount 1 as a surplus gas in a circulation passage, and the existence of airtightness errors in the engine is determined on the basis of the difference between this prescribed amount 1 and the increase in the amount of gas in the circulation passage as detected separately by a gas amount detection means.