A DC-DC converter device and a sub DC-DC converter unit with parallel structure included in the same are disclosed. The DC-DC converter device includes a main DC-DC converter unit configured to receive a (1-1)th reference voltage, a (1-2)th reference voltage and a (n−1)th output voltage, and output an nth first output current corresponding to an nth output voltage; and a sub DC-DC converter unit configured to receive a second reference voltage and the (n−1)th output voltage, and output an nth second output current corresponding to the nth output voltage. Here, an output current in accordance with the nth output voltage corresponds to sum of the nth first output current and the nth second output current.