A battery pack comprising a plurality of battery modules (101, 102), each having two or more battery cells or unit modules, as unit cells, that can be charged and discharged, mounted in a pack case, wherein two or more unit cells constitute one battery module (101, 102), two or more battery modules (101, 102) are arranged in a length direction of the battery pack to constitute one battery module group (110, 120), two battery module groups (110, 120) are symmetric with respect to a coolant discharge part, a coolant inlet port (160, 162) is independently formed at a region of the pack case corresponding to each of the battery modules (101, 102) located at a position opposite to the coolant discharge part, characterized in that: the respective coolant inlet ports (160, 162) are formed such that coolants introduced through the respective coolant inlet ports (160, 162) are not mixed with each other, each of the coolant inlet ports (160, 162) having a width gradually decreased toward a coolant outlet port, the coolant outlet port is formed at a front or a rear of the pack case in the length direction of the battery pack such that coolants introduced through the respective coolant inlet ports (160, 162) cools the unit cells of the respective battery modules (101, 102) while passing through the respective battery modules (101, 102) at the same flow rate per unit time and is then discharged out of the pack case via the coolant discharge part, and each of the coolant inlet ports (160, 162) is formed at the pack case on the opposite side of the coolant outlet port on the basis of a horizontal axis of a corresponding one of the battery modules (101, 102). two or more unit cells constitute one battery module (101, 102), two or more battery modules (101, 102) are arranged in a length direction of the battery pack to constitute one battery module group (110, 120), two battery module groups (110, 120) are symmetric with respect to a coolant discharge part, a coolant inlet port (160, 162) is independently formed at a region of the pack case corresponding to each of the battery modules (101, 102) located at a position opposite to the coolant discharge part, characterized in that: the respective coolant inlet ports (160, 162) are formed such that coolants introduced through the respective coolant inlet ports (160, 162) are not mixed with each other, each of the coolant inlet ports (160, 162) having a width gradually decreased toward a coolant outlet port, the coolant outlet port is formed at a front or a rear of the pack case in the length direction of the battery pack such that coolants introduced through the respective coolant inlet ports (160, 162) cools the unit cells of the respective battery modules (101, 102) while passing through the respective battery modules (101, 102) at the same flow rate per unit time and is then discharged out of the pack case via the coolant discharge part, and each of the coolant inlet ports (160, 162) is formed at the pack case on the opposite side of the coolant outlet port on the basis of a horizontal axis of a corresponding one of the battery modules (101, 102). the respective coolant inlet ports (160, 162) are formed such that coolants introduced through the respective coolant inlet ports (160, 162) are not mixed with each other, each of the coolant inlet ports (160, 162) having a width gradually decreased toward a coolant outlet port, the coolant outlet port is formed at a front or a rear of the pack case in the length direction of the battery pack such that coolants introduced through the respective coolant inlet ports (160, 162) cools the unit cells of the respective battery modules (101, 102) while passing through the respective battery modules (101, 102) at the same flow rate per unit time and is then discharged out of the pack case via the coolant discharge part, and each of the coolant inlet ports (160, 162) is formed at the pack case on the opposite side of the coolant outlet port on the basis of a horizontal axis of a corresponding one of the battery modules (101, 102).