On January 4, 1942, IJN 4th fleet and 1st aviation fleet began to attack Bismarck-Archipel. On 23rd, IJA Nankai (south sea) detachment based on 55th division and IJN landing force occupied Rabaul. This was called Operation R.
Source: History of Showa-era (一億人の昭和史・日本の戦史 8・太平洋戦争 2)(Dec.1978)