An air handler which allows modular assembly at a place of manufacture or outside of a building and modular transportation to a machine room of the building where the air handler will be installed, thereby achieving enhanced transportation convenience. In addition, in the air handler, a plurality of case panels may be assembled with a plurality of module frames via considerably simplified sliding coupling, providing excellent hermetic sealing. As such, manufacturing costs may be reduced due to a reduction in a number of components, and assembly time may be remarkably reduced due to a reduced number of assembly operations. This advantageously results in reduced labor costs and enhanced air conditioning efficiency.