An illumination controller determines a target brightness setting value from a illuminance value measured by a illumination sensor, determines a brightness correction start time in response to a target brightness setting value and a current brightness setting value and a calculation information that are stored in a memory, calculates an amount of brightness setting change per specified time from the current brightness setting value stored in the memory and the target brightness setting value at a brightness correction start time, and outputs an output brightness setting value in response to the amount of brightness setting change that is calculated. The brightness controller adjusts brightness of the display unit in response to the output brightness setting value output from the illumination controller.