A battery comprising: a battery module provided by covering a group of batteries including arranged cells (51, 52, 53, 54) with an insulating external material, the battery module being folded at an area where a conductive member (42a, 42b, 42c) connecting adjacent ones of the cells (51, 52, 53, 54) is located; and a case having a hermetically sealing structure to house the battery module, wherein the case includes first side faces opposite to each other in a first direction, second side faces opposite to each other in a second direction orthogonal to the first direction, a bottom face, and a top face, a pair of extraction electrodes (57, 58) used to extract a power of the battery module to the outside of the case is provided closer to the top face, the number of the cells (51, 52, 53, 54) included in the group of batteries is an even number, and the group of batteries includes a first cell and a second cell, a third cell and a fourth cell, a top end portion of the first cell closer to the top face is connected to one of the pair of extraction electrodes (57, 58) through a first connecting member, a top end portion of the second cell closer to the top face is connected to the other of the pair of extraction electrodes (57, 58) through a second connecting member, a bottom end portion of the third cell closer to the bottom face is connected to a bottom end portion of the first cell closer to the bottom face through the conductive member, a top end portion of fourth cell closer to the top face is connected to a top end portion of the third cell closer to the top face through the conductive member, a bottom end portion of fourth cell closer to the bottom face is connected to a bottom end portion of the second cell closer to the bottom face through the conductive member, the first connecting member is connected to one of a positive electrode and a negative electrode of the first cell, the second connecting member is connected to one of a positive electrode and a negative electrode of the second cell, the conductive member connecting the third cell and the fourth cell is exposed to the outside of external material, the battery further includes; a first voltage detecting portion connected to the top end portion of the first cell and connected to the other of the positive and negative electrodes (51a, 51b) of the first cell; a third voltage detecting portion connected to the top end portion of the second cell and connected to the other of the positive and negative electrodes (52a, 52b) of the second cell; and a second voltage detecting portion connected to an exposed portion of the conductive member connecting the third cell and the fourth cell. a battery module provided by covering a group of batteries including arranged cells (51, 52, 53, 54) with an insulating external material, the battery module being folded at an area where a conductive member (42a, 42b, 42c) connecting adjacent ones of the cells (51, 52, 53, 54) is located; and a case having a hermetically sealing structure to house the battery module, wherein the case includes first side faces opposite to each other in a first direction, second side faces opposite to each other in a second direction orthogonal to the first direction, a bottom face, and a top face, a pair of extraction electrodes (57, 58) used to extract a power of the battery module to the outside of the case is provided closer to the top face, the number of the cells (51, 52, 53, 54) included in the group of batteries is an even number, and the group of batteries includes a first cell and a second cell, a third cell and a fourth cell, a top end portion of the first cell closer to the top face is connected to one of the pair of extraction electrodes (57, 58) through a first connecting member, a top end portion of the second cell closer to the top face is connected to the other of the pair of extraction electrodes (57, 58) through a second connecting member, a bottom end portion of the third cell closer to the bottom face is connected to a bottom end portion of the first cell closer to the bottom face through the conductive member, a top end portion of fourth cell closer to the top face is connected to a top end portion of the third cell closer to the top face through the conductive member, a bottom end portion of fourth cell closer to the bottom face is connected to a bottom end portion of the second cell closer to the bottom face through the conductive member, the first connecting member is connected to one of a positive electrode and a negative electrode of the first cell, the second connecting member is connected to one of a positive electrode and a negative electrode of the second cell, the conductive member connecting the third cell and the fourth cell is exposed to the outside of external material, the battery further includes; a first voltage detecting portion connected to the top end portion of the first cell and connected to the other of the positive and negative electrodes (51a, 51b) of the first cell; a third voltage detecting portion connected to the top end portion of the second cell and connected to the other of the positive and negative electrodes (52a, 52b) of the second cell; and a second voltage detecting portion connected to an exposed portion of the conductive member connecting the third cell and the fourth cell.