A battery cell holder configured so that a cylindrical battery cell is mounted therein, the battery cell holder comprising: an upper cover comprising a first main body to cover an upper part of the battery cell in a state in which a first electrode terminal of the battery cell is exposed and at least one first extension fastening part extending downward from the first main body; and a lower cover comprising a second main body to cover a lower part of the battery cell in a state in which a second electrode terminal of the battery cell having a polarity opposite to that of the first electrode terminal is exposed and at least one second extension fastening part extending upward from the second main body, wherein an end of the at least one first extension fastening part and an end of the at least one second extension fastening part are fastened to each other in a state in which the battery cell is mounted in the battery cell holder, the upper cover and/or the lower cover is provided at an outer side thereof with at least one side fastening part, by which a side of the battery cell holder is fastened to that of another battery cell holder, wherein the at least one side fastening part is configured to have a sliding fastening type groove structure formed in a longitudinal direction of the battery cell holder, wherein the sliding fastening type groove structure comprises a hook and a depressed groove corresponding to the hook in a sectional shape perpendicular to the longitudinal direction, and wherein, when the side fastening part of the battery cell holder is fastened to a side fastening part of anther battery cell holder, a coolant flow channel is formed at a space defined between side fastening parts in the longitudinal direction of battery cells. an upper cover comprising a first main body to cover an upper part of the battery cell in a state in which a first electrode terminal of the battery cell is exposed and at least one first extension fastening part extending downward from the first main body; and a lower cover comprising a second main body to cover a lower part of the battery cell in a state in which a second electrode terminal of the battery cell having a polarity opposite to that of the first electrode terminal is exposed and at least one second extension fastening part extending upward from the second main body, wherein an end of the at least one first extension fastening part and an end of the at least one second extension fastening part are fastened to each other in a state in which the battery cell is mounted in the battery cell holder, the upper cover and/or the lower cover is provided at an outer side thereof with at least one side fastening part, by which a side of the battery cell holder is fastened to that of another battery cell holder, wherein the at least one side fastening part is configured to have a sliding fastening type groove structure formed in a longitudinal direction of the battery cell holder, wherein the sliding fastening type groove structure comprises a hook and a depressed groove corresponding to the hook in a sectional shape perpendicular to the longitudinal direction, and wherein, when the side fastening part of the battery cell holder is fastened to a side fastening part of anther battery cell holder, a coolant flow channel is formed at a space defined between side fastening parts in the longitudinal direction of battery cells.