A battery module includes: cells; and a battery holder made of a resin material that contains a filler in a resin matrix. In the resin material of the battery holder, the lower limit of the content of the filler is determined on the basis of the endothermic amount, and the upper limit of the content of the filler is determined on the basis of the viscosity. Another battery module includes: cylindrical cells; and a battery holder having a plurality of tubular first accommodation portions each having a substantially hexagonal hole and a plurality of tubular second accommodation portions each having a substantially polygonal (heptagonal or more) hole. In the battery holder, the first accommodation portions and second accommodation portions are arranged in a manner of log pile by sandwiching a first column including the first accommodation portions between second columns including the second accommodation portions.