This article analyses the Tokyo High Court’s decision of 18 December 2013 in the M/V Guanabara case, which was Japan’s first case on piracy under the Anti-Piracy Act of 2009. It was an appeal submitted by two Somali pirates, who were seized by US forces and transferred to Japan where they were convicted by the Tokyo District Court for their involvement in a pirate attack against the M/V Guanabara, a Bahamian oil tanker operated by a Japanese company. The article discusses the main holdings of this judgment, which are as follows: first, the Court held that it has adjudicative jurisdiction under customary international law. It argued that the second sentence of Article 105 UNCLOS stipulates a conflict of law rule and does not prohibit a non-seizing state from exercising its adjudicative jurisdiction. Second, the Court found that the transfer of the piracy suspects from the US, a state that does not impose the death penalty for acts of piracy, to Japan, a state that foresees the death penalty as a possible sentence for piracy, did not violate Article 6(1) ICCPR on the right to life. Lastly, with regards to the sentences, the Court sustained the appealed judgment, which took the intermediate value of the upper and lower limits of the range of possible punishments provided for under the Anti-Piracy Act.