The dual type vehicle power-supply apparatus includes a bidirectional power transmission unit capable of performing bidirectional electric power transmission between a high voltage source of a high power-supply voltage supply system and a low voltage source of a low power-supply voltage supply system, and a power transmission controller controlling the bidirectional power transmission unit to perform the bidirectional electric power transmission. The power transmission controller has a function of detecting a surplus amount of electric power in the low power-supply voltage supply system, a function of detecting a shortage amount of electric power in the high power-supply voltage supply system, and a function of commanding the bidirectional power transmission unit to transmit electric power from the low voltage source to the high voltage source by an mount equal to the detected shortage amount when the detected surplus amount is larger than the detected shortage amount.