A display system, a display control method for the display system and a non-transitory computer readable recording medium storing a program executable by a computer coupled to the display device are provided. The display system includes an HMD and a smartphone. The HMD includes an image display unit that transmits outside light and displays an image, and a DP illuminance sensor that detects an illuminance of the outside light. The smartphone includes an SP display control unit that adjusts, based on first illuminance information indicating a detection result of the DP illuminance sensor, a brightness of the image displayed by the image display unit, and an SP illuminance sensor that detects the illuminance. The SP display control unit can adjust the brightness of the image, based on second illuminance information indicating a detection result of the SP illuminance sensor.