Start-up/power generation system of saddle-straddling type motor vehicle

An inverter circuit (61) is connected to a starter/generator (20). The inverter circuit (61) is connected to a battery (7) through a motor line (L1), and is connected to the battery (7) through a generator line (L2). A starter relay (R1) is provided at the motor line (L1), and a charge relay (R2) is provided at the generator line (L2). Further, a DC-DC converter (62) that adjusts a voltage supplied from the inverter circuit (61) to the battery (7) is provided at the generator line (L2), and a load group (G1) is connected to the generator line (L2).