We report two cases in which contrast echocardiography was useful for detecting right-to-left shunt. In case 1, a 53-year-old man was admitted to our hospital after being diagnosed with acute heart failure. Even after improvement of the heart failure, hypoxemia remained. Contrast echocardiography was performed. When contrast medium was injected into the left antecubital vein, it directly drained into the left atrium. When contrast medium was injected into the right antecubital vein, it drained into the right atrium not the left atrium. These findings proved the existence of a right-to-left shunt. In case 2, a 68-year-old man felt dyspnea on mild effort, especially when sitting in an anteflexing posture. In room air, his SpO2 was 95% when sitting in a resting posture and 79% when in an anteflexing sitting posture. Contrast echocardiography was performed. A patent foramen ovale (PFO) was proved using the Valsalva maneuver, and the contrast medium drained from right atrium into the left atrium via the PFO. He underwent patch closure of the PFO, and his symptoms disappeared. Contrast echocardiography should be performed for the diagnosis of chronic hypoxemia for which causes are not detected with routine clinical examinations, in order to confirm right-to-left shunt.