A thermal infrared radiometer indicates a lower sea surface temperature at a large observation angle than the directly measured one. In order to explain this decrease in the apparent temperature, the intensity of the radiation reaching the radiometer is formulated as a function of observation angles of the radiometer. In the equation, we assume that a reflection loss occurs at the sea surface for the radiation from below the surface. The temperature predicted from this formula at the large observation angles fits well to the observed one, indicating that temperatures observed at large observation angles can be corrected