We report a case of flat-infiltrating gallbladder cancer with massive liver invasion. A 64-year-old man was referred to our hospital because a liver mass was detected by medical checkup. A liver mass 45 mm in diameter was shown next to the gallbladder by abdominal computed tomography (CT) and magnetic resonance imaging (MRI), but no remarkable elevated lesion in the gallbladder was shown by these images. Wall thickening and wall abnormality of the gallbladder were also not found. Based on these findings, we suspected intrahepatic cholangiocellular carcinoma (CCC) or liver abscess. Abdominal ultrasonography (US) showed a hyperechoic heterogeneous mass adjacent to the gallbladder. This finding was incompatible with liver abscess. US using a high-frequency probe visualized localized gallbladder wall thickening and continuity of the wall thickening with the liver mass. Contrast-enhanced US (CEUS) revealed hyper-enhancement of the liver mass and gallbladder wall thickening in the arterial phase. Ring enhancement was then shown around the liver mass. The liver mass was depicted as a contrast defect in the post-vascular phase. Based on the CEUS findings, gallbladder cancer was suspected rather than CCC. The final diagnosis was gallbladder cancer with liver invasion, for which surgical resection was performed. Histopathological study revealed that the liver tumor was a poorly to moderately differentiated adenocarcinoma. In the gallbladder, adenocarcinoma cells had spread widely, compatible with flat-infiltrating gallbladder cancer. In conclusion, US using a high-frequency probe would be a very useful to confirm the relation between liver tumors near the gallbladder and the wall of the gallbladder. And CEUS will be helpful to diagnose liver tumors adjacent to the gallbladder.