Abstract: An indoor unit of an air conditioning system includes front and rear cases joined together to form a space in which a heat exchanger, fan, and fan motor are housed. A grill member hingedly mounted on the front case has a front air intake opening formed therein. A top air intake opening is formed in an upper side of the rear case. A filter is mounted on a rear side of the grill member to extend across the front air intake opening. An upper portion of the filter underlies the top air intake opening. Edges of the front and rear case overlap one another, and one of those edges forms a slot in which the other edge is received. Boards for carrying electrical components are mounted in an out-of-way manner to minimize the space taken up.