It is well known that the advection fog invades from the sea to the Pacific coast of the eastern part of Hokkaido in summer season. The summer climate of this region is characterized by the fog, which brings lower temperature, the shorter sunshine dura-tion, and the higher humidity. In this study, the present writer tries to clarify the seasonal and diurnal variation of fog frequency, the speeds of fog invasion and dis-sipation, and other climatological factors during the invasion or dissipation of fog.<br> The results are as follows:<br> 1. In Kushiro District, fog frequency becomes higher from the 32nd half decade to the 47th. Especially the 33rd to 36th and the 41st to 43rd are the most frequent. In these periods the pressure pattern of the Baiu type and the summer type prevail. This variation is shown in Pig. 2.<br> 2. Fog frequency has great diurnal changes. Generally, it is the most frequent during 5:00 to 6:00 in the morning, and reaches over 40% at its peak. The diurnl change of fog frequency diminishes on the sea shore, and increases in the inland, where the fog invades at night and dissipates early in the morning, as shown in Fig. 3. The fog in the inland is characterized by the radiation type, but mixed with the advection type.<br> 3. The time of fog dissipation is mostly between 7:00 to 8:00 in the morning, and has a little seasonal fluctuation. The speed of fog front retreat is about 2.5km/h, but the higher speed is observed on the seashore in contrast to the lower speed in the inland.<br> On the other hand, fog invasion occurs mostly between 19:00 to 22:00 with the average speed of 2.0km/h. It is lower on the seashore and higher in the inland.<br> 4. In the daytime (9:00-15:00), the invasion of shallow advection fog makes the air temperature lower by 1. 5°C to 2. 3°C on an average and the relative humidity higherr by 5 to 12% for 30 to 70 minutes.<br> 5. The temperature distribution patterns during the time of fog invasion are shown in Figs. a-, -10. The difference of temperatures between on the seashore and in the inland (6.0km from the coastal line) amount to 5 or 6°C at maximum, but it is only about 2°C after the fog dissipation.<br> 6. The fog front marches from Shinto Cape, southern part of Kushiro Port, or from the eastern coast of Kushiro District to the inland. It invades faster in the eastern and western parts of the New Kushiro River, but slowly in the highly urbanized area, the leeward area of upland. The air temperature of this area is higher even during the time of fog invasion.<br> 7. In summer, the winds of this district blow mostly from the sea to the inland. This is also true at the time of the fog invasion. As given in Figs. 8_??_10, the wind during the time of the fog invasion blows at right angles to the isothermal line. This means that the cold wet air mass advects to this area from sea surface. Fig. 11 shows wind roses during the time of the fog invasion in the day time. The average wind speed is 4.5m/sec on the coast and 3.0m/sec in the inland.<br> 8. In order to clarify the wind conditions in detail, the wind shaped trees as an indicator were observed in this district. The distribution of wind conditions as revealed by the wind shaped trees shown in Fig. 12 resembles to that of the results obtained by the instrumental observation shown in Fig. 11.