In order to clarify the main factor of the periodical aerodynamic noise in the off-design point of the propeller fan, the relative flow field in the blade tip is measured by the hot-wire anemometer with the phase-lock method. In particular, we discuss the influence of the rotational speed of the impeller experimentally for clarifying the relationship between the relative flow field and the periodical aerodynamic noise. When the propeller fan is operated at the off-design point, the periodaical aerodynamic noise is generated in the lower frequency domain than that of the blade passing frequency. The energy of the velocity fluctuation in the leading edge of the blade tip became large at the same frequency band of the periodical aerodynamic noise. We indicated experimentally that the leakage flow at the leading edge in the relative flow did not interfere to the next blade. These experimental results indicate the periodical aerodynamic noise was generated because the pressure fluctuation due to the leakage flow became strong.