A battery pack comprising a battery cell assembly and a pack case (10, 20) in which the battery cell assembly is disposed: the battery cell assembly comprising: (a) a battery cell array comprising two or more battery cells, each of which has an electrode assembly of a cathode/separator/anode structure disposed in a battery case together with an electrolyte in a sealed state, arranged in a lateral direction; and (b) a protection circuit module connected to an upper end of the battery cell array to control an operation of the battery cell array, wherein outer sides of the battery cells or an outer side of the battery cell array is coated with a resin by insert injection molding excluding electrode terminals of the battery cells, the pack case (10, 20) comprising: a lower case at which the battery cell array and the protection circuit module are disposed and an upper case to cover the lower case so as to fix the battery cell array and the protection circuit module in place, wherein: the lower case is partitioned into a battery cell mounting part, at which the battery cells are disposed, and a protection circuit module mounting part, at which the protection circuit module is disposed, characterized in that a partition wall is formed at an interface between the battery cell mounting part and the protection circuit module mounting part, while openings, through which electrode terminals of the battery cells are exposed toward the protection circuit module, are formed at portions of the partition wall corresponding to electrical connection regions between the electrode terminals of the battery cells and the protection circuit module, and the protection circuit module mounting part comprises support parts to support the electrical connection regions between the electrode terminals of the battery cells and the protection circuit module. the battery cell assembly comprising: (a) a battery cell array comprising two or more battery cells, each of which has an electrode assembly of a cathode/separator/anode structure disposed in a battery case together with an electrolyte in a sealed state, arranged in a lateral direction; and (b) a protection circuit module connected to an upper end of the battery cell array to control an operation of the battery cell array, wherein outer sides of the battery cells or an outer side of the battery cell array is coated with a resin by insert injection molding excluding electrode terminals of the battery cells, (a) a battery cell array comprising two or more battery cells, each of which has an electrode assembly of a cathode/separator/anode structure disposed in a battery case together with an electrolyte in a sealed state, arranged in a lateral direction; and (b) a protection circuit module connected to an upper end of the battery cell array to control an operation of the battery cell array, wherein outer sides of the battery cells or an outer side of the battery cell array is coated with a resin by insert injection molding excluding electrode terminals of the battery cells, the pack case (10, 20) comprising: a lower case at which the battery cell array and the protection circuit module are disposed and an upper case to cover the lower case so as to fix the battery cell array and the protection circuit module in place, wherein: the lower case is partitioned into a battery cell mounting part, at which the battery cells are disposed, and a protection circuit module mounting part, at which the protection circuit module is disposed, characterized in that a partition wall is formed at an interface between the battery cell mounting part and the protection circuit module mounting part, while openings, through which electrode terminals of the battery cells are exposed toward the protection circuit module, are formed at portions of the partition wall corresponding to electrical connection regions between the electrode terminals of the battery cells and the protection circuit module, and the protection circuit module mounting part comprises support parts to support the electrical connection regions between the electrode terminals of the battery cells and the protection circuit module. the lower case is partitioned into a battery cell mounting part, at which the battery cells are disposed, and a protection circuit module mounting part, at which the protection circuit module is disposed, characterized in that a partition wall is formed at an interface between the battery cell mounting part and the protection circuit module mounting part, while openings, through which electrode terminals of the battery cells are exposed toward the protection circuit module, are formed at portions of the partition wall corresponding to electrical connection regions between the electrode terminals of the battery cells and the protection circuit module, and the protection circuit module mounting part comprises support parts to support the electrical connection regions between the electrode terminals of the battery cells and the protection circuit module.