A terminal base of a power supply device for an electric vehicle, the power supply device including a battery case for accommodating battery cells for supplying electric power to an electric motor of an electric vehicle through female-side terminals, and the power supply device being mountable to and detachable from the electric vehicle, wherein the terminal base comprises: a base having a substantially rectangular opening for accepting a substantially rectangular insulator board holding a plurality of male-side terminals; a first lower guide projecting toward the opening along a lower surface of the base at one edge of the opening; a first upper guide projecting toward the opening along an upper surface of the base from the edge from which the first lower guide projects; a second lower guide projecting toward the opening along the lower surface of the base at an edge opposed to the edge from which the first lower guide and the first upper guide project; and a second upper guide projecting toward the opening along the upper surface of the base from the edge from which the second lower guide projects, wherein a first edge of the insulator board is clamped by the first lower guide and the first upper guide from lower and upper sides, and a second edge of the insulator board opposed to the first edge of the insulator board is clamped by the second lower guide and the second upper guide from the lower and upper sides, whereby the insulator board is supported in a floating state, wherein at the time of connecting the female-side terminals on the battery case to the male-side terminals, it is ensured that even if the relative positions of the male-side terminals and the female-side terminals are deviated from the properly facing positions, a positional discrepancy is absorbed by movement of the male-side terminals relative to the terminal base; wherein the directions and sides are with respect to an electric vehicle in use. a base having a substantially rectangular opening for accepting a substantially rectangular insulator board holding a plurality of male-side terminals; a first lower guide projecting toward the opening along a lower surface of the base at one edge of the opening; a first upper guide projecting toward the opening along an upper surface of the base from the edge from which the first lower guide projects; a second lower guide projecting toward the opening along the lower surface of the base at an edge opposed to the edge from which the first lower guide and the first upper guide project; and a second upper guide projecting toward the opening along the upper surface of the base from the edge from which the second lower guide projects, wherein a first edge of the insulator board is clamped by the first lower guide and the first upper guide from lower and upper sides, and a second edge of the insulator board opposed to the first edge of the insulator board is clamped by the second lower guide and the second upper guide from the lower and upper sides, whereby the insulator board is supported in a floating state, wherein at the time of connecting the female-side terminals on the battery case to the male-side terminals, it is ensured that even if the relative positions of the male-side terminals and the female-side terminals are deviated from the properly facing positions, a positional discrepancy is absorbed by movement of the male-side terminals relative to the terminal base; wherein the directions and sides are with respect to an electric vehicle in use.