To provide an indoor unit for an air conditioner capable of preventing dew condensation at a back face of the indoor unit, dispensing with a drain trap structure, and reducing a thickness of a main body casing as thin as possible by using a vacuum heat insulating material of very high heat insulating performance for a foamed polystyrene heat insulating material disposed inside the indoor unit.

In this indoor unit 1 for the air conditioner comprising box-shaped indoor unit casings 2, 3, 4, an air suction opening 5 formed on a central portion at a front face side, a pair of right and left air supply openings 7 formed right and left at the front face side, a pair of air passages 6 formed inside and continued to the pair of right and left air supply openings 7 from the air suction opening 5, a common fan positioned at an upstream side of each of the air passages 6 corresponding to the air suction openings, and a pair of right and left heat exchangers 9 positioned at a downstream side of the pair of air passages 6 and disposed corresponding to the pair of right and left air supply openings 7, the flat vacuum heat insulating material 10 vacuum inside is disposed on inner wall face parts corresponding to the pair of right and left heat exchangers 9 of the indoor unit casings 2, 3, 4.