An air conditioner comprising: an outdoor unit; an indoor unit communicating with the outdoor unit; and a liquid-stay preventing device (310, 320) configured to heat and evaporate a liquefied refrigerant so as to prevent liquefied refrigerant circulating between the indoor unit and the outdoor unit from being accumulated at a low pressure side including the indoor unit, characterized in that the air conditioner is a multi-type air conditioner having a plurality of indoor units, whereby the liquid-stay preventing device (310, 320) configured to heat and evaporate a liquefied refrigerant so as to prevent liquefied refrigerant circulating between the indoor units and the outdoor unit from being accumulated at the low pressure side including the indoor units, the liquid-stay preventing device comprises: a tank body; a heat exchange part installed inside the tank body for heating a liquefied refrigerant accumulated therein in order to evaporate the refrigerant; and connection pipes connecting the tank body and the heat exchange part to the indoor units and the outdoor unit a liquefied refrigerant level detecting sensor installed inside the tank body, for detecting a level of liquefied refrigerant, and generating a signal; and a hot gas opening/closing valve installed on a first connection pipe, for opening or closing the first connection pipe so as to allow the refrigerant to flow to the heat exchange part or prevent the flowing to the heat exchange part; and a controller for determining and indicating whether to open or close the hot gas opening/closing valve according to the signal. an outdoor unit; an indoor unit communicating with the outdoor unit; and a liquid-stay preventing device (310, 320) configured to heat and evaporate a liquefied refrigerant so as to prevent liquefied refrigerant circulating between the indoor unit and the outdoor unit from being accumulated at a low pressure side including the indoor unit, characterized in that the air conditioner is a multi-type air conditioner having a plurality of indoor units, whereby the liquid-stay preventing device (310, 320) configured to heat and evaporate a liquefied refrigerant so as to prevent liquefied refrigerant circulating between the indoor units and the outdoor unit from being accumulated at the low pressure side including the indoor units, the liquid-stay preventing device comprises: a tank body; a heat exchange part installed inside the tank body for heating a liquefied refrigerant accumulated therein in order to evaporate the refrigerant; and connection pipes connecting the tank body and the heat exchange part to the indoor units and the outdoor unit a liquefied refrigerant level detecting sensor installed inside the tank body, for detecting a level of liquefied refrigerant, and generating a signal; and a hot gas opening/closing valve installed on a first connection pipe, for opening or closing the first connection pipe so as to allow the refrigerant to flow to the heat exchange part or prevent the flowing to the heat exchange part; and a controller for determining and indicating whether to open or close the hot gas opening/closing valve according to the signal. a tank body; a heat exchange part installed inside the tank body for heating a liquefied refrigerant accumulated therein in order to evaporate the refrigerant; and connection pipes connecting the tank body and the heat exchange part to the indoor units and the outdoor unit a liquefied refrigerant level detecting sensor installed inside the tank body, for detecting a level of liquefied refrigerant, and generating a signal; and a hot gas opening/closing valve installed on a first connection pipe, for opening or closing the first connection pipe so as to allow the refrigerant to flow to the heat exchange part or prevent the flowing to the heat exchange part; and a controller for determining and indicating whether to open or close the hot gas opening/closing valve according to the signal.