There are several methods for calculating the length of a capillary tube, but the mathematical results do not necessarily agree with the experimental data. In order to find the cause, the authors have observed the refrigerant flow in a glass-capillary tube visually and have reconfirmed it to be homogeneous. The pressure and temperature distribution along the capillary tube were measured and we recognized that the starting point of vaporization became delayed. We found that a theoretically calculated pressure distribution, presupposing adiabatic and homogeneous flow, agreed accurately with measured values, if the delay of vaporization was estimated correctly. Using the theoretical equation, we developed a simple method for calculating the length of the refrigerant vaporizing flow region in the capillary tube. We believe that this method will be a useful tool for designing new refrigerating systems. In addition, valuable knowledge about the refrigerant flow in the capillary tube was obtained.