A display brightness control circuit of a liquid crystal display device includes a logarithmic-linear converting circuit for converting an illuminance signal having a logarithmic relationship with incident light into a linear signal. When an output of an external light sensor is a logarithmic signal, the logarithmic signal is converted into a linear signal through the logarithmic-linear converting circuit, and when an output of the external light sensor is a linear signal, the logarithmic-linear converting circuit is not involved. The logarithmic-linear converting circuit corresponds to an exponential circuit which is an inverse function of a logarithm log. Further, when even the same logarithmic output illuminance sensor is different in input-output characteristics, setting of the abovementioned exponential circuit can be changed.