A power transmission device includes first and second drive shafts, a differential mechanism, a case, a pinion shaft, a pinion gear, and a bearing. The differential mechanism is connected to the second drive shaft. The differential mechanism includes a first side gear connected to the first drive shaft, a second side gear, first and second pinion mate gears that engage with the first and second side gears, first and second pinion mate shafts that respectively support the first and second pinion mate gears. The pinion gear includes small and large diameter gear parts. The bearing is positioned between the pinion shaft and the pinion gear, and is lubricated via an oil hole that is open facing an outside of the case at a small diameter gear part side. The small diameter gear part is positioned between the first and second pinion mate shafts in a circumferential direction.