A battery unit, comprising: a plurality of battery cells; a cooling substrate including cooling surfaces which are provided on both opposite sides thereof, the battery cells being disposed so as to hold the cooling substrate on the both sides of the cooling substrate, an opposite-to-terminal face of the battery cell opposite to a terminal face thereof where negative and positive terminals are provided being attached to the cooling surface of the cooling substrate so as to transfer heat between the cooling surface and the opposite-to-terminal face attached thereto, a holding member which includes an assembling portion assembled to an end portion of a side of the opposite-to-terminal face of the battery cell, the holding member being provided between the cooling substrate and the battery cell, the holding member is formed with an opening portion at a position corresponding to the battery cell, and includes a holding piece which is raised substantially perpendicularly from the cooling substrate so as to surround the opening portion, characterized in that a deformable heat transfer member is accommodated in the opening portion so as to be interposed between the opposite-to-terminal face and the cooling substrate, the holding piece is formed with a sloping portion which contacts a lateral face of the battery cell which is connected with the opposite-to-terminal face so as to hold the end portion of the side of the opposite-to-terminal face of the battery cell, and the sloping portion is plastically deformed so that the battery cell is held in a position where the battery cell is forced into the assembling portion. a plurality of battery cells; a cooling substrate including cooling surfaces which are provided on both opposite sides thereof, the battery cells being disposed so as to hold the cooling substrate on the both sides of the cooling substrate, an opposite-to-terminal face of the battery cell opposite to a terminal face thereof where negative and positive terminals are provided being attached to the cooling surface of the cooling substrate so as to transfer heat between the cooling surface and the opposite-to-terminal face attached thereto, a holding member which includes an assembling portion assembled to an end portion of a side of the opposite-to-terminal face of the battery cell, the holding member being provided between the cooling substrate and the battery cell, the holding member is formed with an opening portion at a position corresponding to the battery cell, and includes a holding piece which is raised substantially perpendicularly from the cooling substrate so as to surround the opening portion, characterized in that a deformable heat transfer member is accommodated in the opening portion so as to be interposed between the opposite-to-terminal face and the cooling substrate, the holding piece is formed with a sloping portion which contacts a lateral face of the battery cell which is connected with the opposite-to-terminal face so as to hold the end portion of the side of the opposite-to-terminal face of the battery cell, and the sloping portion is plastically deformed so that the battery cell is held in a position where the battery cell is forced into the assembling portion.