In an evaporator for evaporating mists of liquid raw material to thereby generate start gas for layer-formation, an evaporator body has an evaporator chamber defined therein, and a mist supply throat for introducing the mists into the evaporating chamber. The evaporator chamber is defined by a principal evaporating face which opposes to the mist supply throat. The evaporator body also has a start-gas supply passage which is formed therein between the mist supply throat and the principal evaporating face such that the start gas flows out of the evaporating chamber through the start-gas supply passage. A ridge member is provided on an inner side wall surface of the evaporating chamber between the start-gas is supply passage and the principal evaporating face so that a tip edge of the ridge member is directed to the principal evaporating face.