It is well known that the terrestrial temperature varies with the period of 24 hours mainly due to the rotation of the earth in the field of the solar radiation. However, we observed the phenomenon that the semidiual (12 hour) temperature oscillation is dominant in our underground laboratory. By the spectrum analysis, the close correlation between the variations of tenperature and atmospheric pressure was discovered, and the result of phase analysis showed that the phase of semidiual pressure lags behind that of the semidiual temperature.A model of heat conduction between air and rock is presented to explain the semidiual temperature oscillation observed in the underground laboratory.