An air conditioner comprising: an electric dust collecting device for electrically charging dust in air and collecting the charged dust; and a blower for blowing air toward the electric dust collecting device, wherein the electric dust collecting device comprises: a first filter unit including a plurality of discharge electrode plates (11 - 15), the discharge electrode plates being spaced apart from each other by a space (S1 - S4) defined therebetween; a high voltage generator electrically connected to each one of the plurality of discharge electrode plates (11 - 15); and a second filter unit (4, 4') disposed downstream of the first filter unit in a flow direction of air blown by the blower and connected to the ground to cause corona discharge between the discharge electrode plates (11 - 15) and the second filter unit (4, 4'), whereby particles electrically charged by corona discharge are collected at the second filter unit (4, 4'), characterized in that the electric dust collecting device further includes a first filter case in which the plurality of discharge electrode plates (11 - 15) is installed, wherein the first filter case is provided with a second filter case attachment on which the second filter unit is detachably mounted, wherein the second filter case attachment is constituted by a sliding guide such that the second filter unit is deteachably mounted on the first filter case in a sliding manner, and wherein the electric dust collecting device further comprises: a first earth line connected to the second filter unit (4, 4'); a first earth terminal connected to the first earth line; a second earth line connected to the ground; and a second earth terminal connected to the second earth line and adapted to contact and be disconnected from the first earth terminal. an electric dust collecting device for electrically charging dust in air and collecting the charged dust; and a blower for blowing air toward the electric dust collecting device, wherein the electric dust collecting device comprises: a first filter unit including a plurality of discharge electrode plates (11 - 15), the discharge electrode plates being spaced apart from each other by a space (S1 - S4) defined therebetween; a high voltage generator electrically connected to each one of the plurality of discharge electrode plates (11 - 15); and a second filter unit (4, 4') disposed downstream of the first filter unit in a flow direction of air blown by the blower and connected to the ground to cause corona discharge between the discharge electrode plates (11 - 15) and the second filter unit (4, 4'), whereby particles electrically charged by corona discharge are collected at the second filter unit (4, 4'), a first filter unit including a plurality of discharge electrode plates (11 - 15), the discharge electrode plates being spaced apart from each other by a space (S1 - S4) defined therebetween; a high voltage generator electrically connected to each one of the plurality of discharge electrode plates (11 - 15); and a second filter unit (4, 4') disposed downstream of the first filter unit in a flow direction of air blown by the blower and connected to the ground to cause corona discharge between the discharge electrode plates (11 - 15) and the second filter unit (4, 4'), whereby particles electrically charged by corona discharge are collected at the second filter unit (4, 4'), characterized in that the electric dust collecting device further includes a first filter case in which the plurality of discharge electrode plates (11 - 15) is installed, wherein the first filter case is provided with a second filter case attachment on which the second filter unit is detachably mounted, wherein the second filter case attachment is constituted by a sliding guide such that the second filter unit is deteachably mounted on the first filter case in a sliding manner, and wherein the electric dust collecting device further comprises: a first earth line connected to the second filter unit (4, 4'); a first earth terminal connected to the first earth line; a second earth line connected to the ground; and a second earth terminal connected to the second earth line and adapted to contact and be disconnected from the first earth terminal. a first earth line connected to the second filter unit (4, 4'); a first earth terminal connected to the first earth line; a second earth line connected to the ground; and a second earth terminal connected to the second earth line and adapted to contact and be disconnected from the first earth terminal.