The osmotic resistance of human erythrocytes under the hypotonic condition is studied. We experimentally observed the swelling and hemolytic behavior of a human erythrocyte caused by a hypotonic solution. The volume change by swelling was approximately estimated from the two-dimensional images of the erythrocyte during the experiments. The results show that the erythrocytes swell until getting spherical shape, and they tend to be reduced in volume and finally hemolyzed. Based on the light absorption characteristics, the mass of the hemoglobin included in an erythrocyte was estimated. Although the hemoglobin mass in an erythrocyte does not change immediately after the hypotonic solution was added, it starts to decrease at a certain time, indicating that the hemoglobin starts to leach out of the erythrocytes due to the hemolysis. The onset time would coincide with the time at which the erythrocyte volume starts to decrease. It is found that the hemoglobin leaching would begin after the erythrocyte getting spherical and it would cause the volume decrease. Additionally, we have attempted to visualize the hemoglobin concentration both in and out of the erythrocyte. The time-dependent concentration change was captured.