An indoor unit for an air conditioner, comprising: a heat exchanger; a drain pan laid out at a lower side of the heat exchanger; and a drain pump that discharges water collected in the drain pan to an outside, wherein the drain pan has: a drain receiver that is extended along the heat exchanger and that receives the water generated on a surface of the heat exchanger, at the lower side of the heat exchanger; and a partition wall that partitions an inside of the drain receiver into a region on an upstream side and a region on a downstream side of an airflow so as to form an inner groove which receives the water in the region on the upstream side of the airflow and an outer groove which receives the water in the region on the downstream side of the airflow, characterized in that: a bottom of the outer groove and a bottom of the inner groove have respectively inclinations which are descending slopes where the water flows toward the drain pump, wherein the partition wall is erected to be in contact with the heat exchanger from a lower side, and an inclination angle of the inclination of the bottom of the outer groove is larger than an inclination angle of the inclination of the bottom of the inner groove. a heat exchanger; a drain pan laid out at a lower side of the heat exchanger; and a drain pump that discharges water collected in the drain pan to an outside, wherein the drain pan has: a drain receiver that is extended along the heat exchanger and that receives the water generated on a surface of the heat exchanger, at the lower side of the heat exchanger; and a partition wall that partitions an inside of the drain receiver into a region on an upstream side and a region on a downstream side of an airflow so as to form an inner groove which receives the water in the region on the upstream side of the airflow and an outer groove which receives the water in the region on the downstream side of the airflow, characterized in that: a bottom of the outer groove and a bottom of the inner groove have respectively inclinations which are descending slopes where the water flows toward the drain pump, wherein the partition wall is erected to be in contact with the heat exchanger from a lower side, and an inclination angle of the inclination of the bottom of the outer groove is larger than an inclination angle of the inclination of the bottom of the inner groove. a drain receiver that is extended along the heat exchanger and that receives the water generated on a surface of the heat exchanger, at the lower side of the heat exchanger; and a partition wall that partitions an inside of the drain receiver into a region on an upstream side and a region on a downstream side of an airflow so as to form an inner groove which receives the water in the region on the upstream side of the airflow and an outer groove which receives the water in the region on the downstream side of the airflow, characterized in that: a bottom of the outer groove and a bottom of the inner groove have respectively inclinations which are descending slopes where the water flows toward the drain pump, wherein the partition wall is erected to be in contact with the heat exchanger from a lower side, and an inclination angle of the inclination of the bottom of the outer groove is larger than an inclination angle of the inclination of the bottom of the inner groove. a bottom of the outer groove and a bottom of the inner groove have respectively inclinations which are descending slopes where the water flows toward the drain pump, wherein the partition wall is erected to be in contact with the heat exchanger from a lower side, and an inclination angle of the inclination of the bottom of the outer groove is larger than an inclination angle of the inclination of the bottom of the inner groove.