We report that the transversus abdominis plane block (TAP block) can be performed under ultrasound guidance using not a linear probe but a convex probe in a markedly obese patient undergoing laparoscopy-assisted distal gastrectomy. The TAP block is effective for providing perioperative analgesia. The common probe for the TAP block is a high-frequency linear probe, which can not depict the deeper tissues. We used a low-frequency convex probe for TAP block, which clearly showed the spread of local anesthetics in TAP block in a markedly obese patient. A convex probe is preferable for TAP block in markedly obese patients.