A battery pack comprising: a unit cell that has a cell body including a power-generating element and being formed to have a flat shape, and an electrode tab extending out from the cell body, wherein the electrode tab has a proximal end portion on the cell body side and a distal end portion, and the distal end portion of the electrode tab is bent, wherein the electrode tab has a hole or notch, and the hole or notch on the electrode tab is formed in an elongated shape from the proximal end portion to the distal end portion; and a spacer that includes a protrusion formed in a protruding shape that is inserted into the hole or notch provided on the electrode tab to guide the electrode tab, a first recess formed in a recessed shape so as to be separated from the electrode tab around the base of the protrusion, and an opening that allows a portion of the first recess to communicate with a side surface of the spacer, wherein the side surface is an abutting surface of the spacer that faces and contacts the distal end portion of the electrode tab; characterized in that the spacer further includes a second recess formed in a recessed shape; and a gap extending from the first recess and the second recess to the side surface between the spacer and the unit cell functions as a drainage channel. a unit cell that has a cell body including a power-generating element and being formed to have a flat shape, and an electrode tab extending out from the cell body, wherein the electrode tab has a proximal end portion on the cell body side and a distal end portion, and the distal end portion of the electrode tab is bent, wherein the electrode tab has a hole or notch, and the hole or notch on the electrode tab is formed in an elongated shape from the proximal end portion to the distal end portion; a cell body including a power-generating element and being formed to have a flat shape, and an electrode tab extending out from the cell body, wherein the electrode tab has a proximal end portion on the cell body side and a distal end portion, and the distal end portion of the electrode tab is bent, wherein the electrode tab has a hole or notch, and the hole or notch on the electrode tab is formed in an elongated shape from the proximal end portion to the distal end portion; and a spacer that includes a protrusion formed in a protruding shape that is inserted into the hole or notch provided on the electrode tab to guide the electrode tab, a first recess formed in a recessed shape so as to be separated from the electrode tab around the base of the protrusion, and an opening that allows a portion of the first recess to communicate a protrusion formed in a protruding shape that is inserted into the hole or notch provided on the electrode tab to guide the electrode tab, a first recess formed in a recessed shape so as to be separated from the electrode tab around the base of the protrusion, and an opening that allows a portion of the first recess to communicate with a side surface of the spacer, wherein the side surface is an abutting surface of the spacer that faces and contacts the distal end portion of the electrode tab; characterized in that the spacer further includes a second recess formed in a recessed shape; and a gap extending from the first recess and the second recess to the side surface between the spacer and the unit cell functions as a drainage channel.