In a battery unit 10 in which plural high-voltage batteries 31a to 33a are accommodated in a battery case 50, the battery case 50 includes: a bottom plate 51A on which the batteries 31a to 33a are mounted; and a cover 52 covering the batteries 31a to 33a from above. The bottom plate 51A includes a tray 54 having a plate shape, lengthwise reinforcing members 55, and brackets 53. The lengthwise reinforcing members 55 are disposed so as to connect at least adjacent brackets 53. The lengthwise reinforcing members 55 and the brackets 53 are disposed in a lattice shape with the tray 54 interposed therebetween. The batteries 31a to 33a are fixed to the lengthwise reinforcing members 55 such that a longitudinal direction thereof faces the vehicle width direction. The battery case 50 is fixed under a floor of a vehicle V by the brackets 53.