An air-conditioner casing for an air conditioner mounted on a ceiling of a bus are provided. The air-conditioner casing includes: a pair of bases having first and second bases which are installed to be spaced apart from and opposite to each other; and one or more shrouds coupled in a space between the first and second bases, in which the number of shrouds to be connected and installed in the casing may be determined based on a capacity of an air conditioner.