Head-up display system and moving body equipped with head-up display system

A head-up display system configured to project an image on a display member disposed in an eye direction of an observer, the head-up display system comprising:

• a display device configured to display the image;
• and
• a projection optical system configured to project on the display member the image displayed on the display device, wherein
• the projection optical system includes,
• a mirror ML configured to reflect a light beam toward the display member, and
• a mirror ML−1 configured to reflect the light beam toward mirror ML,
• the mirror ML and the mirror ML−1 being arranged in a stated order in a direction from the display member to the display device,
• the display device is positioned under the mirror ML−1, and
• the mirror ML is positioned on a side opposite to the observer with respect to the mirror ML−1, and satisfies following condition (1)
• 0.01<|MLD/MLW|×|ML−1Z/ML−1W| (1)
• where
• MLD is a depth of the mirror ML
• MLW is a lateral-dimension of the mirror ML
• ML−1Z is a maximum sag amount of the mirror ML−1
• ML−1W is a lateral-dimension of the mirror ML−1.