A battery pack configured to be mounted to a battery pack mounting part of a device for supplying power thereto, comprising: a base plate including one surface on which two or more battery module assemblies (141, 142, 143) respectively having a plurality of battery cells arranged therein are mounted in a separated state from each other with a predetermined interval therebetween; a cover member mounted on one surface of the base plate while enclosing the battery module assemblies (141, 142, 143); and a reinforcement supporting member (131, 132) positioned at a separation part between the battery module assemblies (141, 142, 143) while supporting the mounting state of the cover member for the base plate, wherein - the battery pack is configured to be mounted in a state in which an external circumferential surface of the cover member faces the battery pack mounting part of the device, characterized in that, - the reinforcement supporting member (131, 132) is formed of a structure in which a connect beam is connected between at least two supporting parts, - a mounting space (114, 115, 116) for mounting each of the battery module assemblies (141, 142, 143) is formed of the recessed structure at one surface of the base plate, and - a recessed depth of the mounting space (114, 115, 116) of the battery module assembly (141, 142, 143) is formed of a size of 10 % to 50 % with reference to the height corresponding to the mounting direction of the battery module assembly (141, 142, 143) to prevent a displacement of the battery module assembly (141, 142, 143). a base plate including one surface on which two or more battery module assemblies (141, 142, 143) respectively having a plurality of battery cells arranged therein are mounted in a separated state from each other with a predetermined interval therebetween; a cover member mounted on one surface of the base plate while enclosing the battery module assemblies (141, 142, 143); and a reinforcement supporting member (131, 132) positioned at a separation part between the battery module assemblies (141, 142, 143) while supporting the mounting state of the cover member for the base plate, wherein - the battery pack is configured to be mounted in a state in which an external circumferential surface of the cover member faces the battery pack mounting part of the device, characterized in that, - the reinforcement supporting member (131, 132) is formed of a structure in which a connect beam is connected between at least two supporting parts, - a mounting space (114, 115, 116) for mounting each of the battery module assemblies (141, 142, 143) is formed of the recessed structure at one surface of the base plate, and - a recessed depth of the mounting space (114, 115, 116) of the battery module assembly (141, 142, 143) is formed of a size of 10 % to 50 % with reference to the height corresponding to the mounting direction of the battery module assembly (141, 142, 143) to prevent a displacement of the battery module assembly (141, 142, 143). - the battery pack is configured to be mounted in a state in which an external circumferential surface of the cover member faces the battery pack mounting part of the device, characterized in that, - the reinforcement supporting member (131, 132) is formed of a structure in which a connect beam is connected between at least two supporting parts, - a mounting space (114, 115, 116) for mounting each of the battery module assemblies (141, 142, 143) is formed of the recessed structure at one surface of the base plate, and - a recessed depth of the mounting space (114, 115, 116) of the battery module assembly (141, 142, 143) is formed of a size of 10 % to 50 % with reference to the height corresponding to the mounting direction of the battery module assembly (141, 142, 143) to prevent a displacement of the battery module assembly (141, 142, 143).