We report obstruction of the left ventricle outflow tract (LVOT) caused by cardiac myxoma that was misidentified as an accessory mitral valve tissue preoperatively. A 65-year-old woman presented with chest discomfort that persisted for 7 days. Transthoracic echocardiography (TTE) revealed a mobile, low-echogenic, balloon-shaped mass attached to the anterior mitral valve leaflet and papillary muscle, which was suspected to be an accessory mitral valve tissue. Because the mass caused LVOT obstruction and it could result in hemodynamic instability, emergency operation was performed. Intraoperative transesophageal echocardiography (TEE) was performed, and the mass had irregular margins and was pedunculated, with a stalk originating from the left ventricle (LV) wall and extending to the lateral chordae of the mitral valve. The surgeon excised the mass filled with the myxomatous mass, which was yellowish and gelatinous and had a stiff stalk, and histopathologic diagnosis confirmed a myxoma. Although mitral valve or LV myxomas are rare, TEE is a useful tool for distinguishing a myxoma from other intracardiac masses, such as vegetation or an accessory mitral valve tissue.