The accident involving the operated by MOL's Mitsui OSK Passenger Line ship took place on the night of December 30. Both the pier and the vessel were damaged.
None of some 600 people onboard, of which 372 passengers, were injured.
According to Transport ministry officials, a breath test performed by authorities 4 to 5 hours after the accident found that the alcohol level of the captain exceeded the limit of 0.15 milligrams/liter.
The captain was quoted as saying that he drank following the accident to calm down.
The 22,400-ton Nippon Maru is one of Japan's most luxurious passenger ships.