Light irradiation device and light irradiation system

A light irradiation device comprising:

• an optical fiber;
• and
• a light transmissive optical path changing member disposed on a distal side of the light irradiation device relative to the optical fiber and configured to change an optical path of light entering the optical path changing member from the optical fiber, wherein
• the optical path changing member includes a proximal side member disposed on a proximal side of the optical path changing member and a distal side member disposed on a distal side of the optical path changing member,
• the proximal side member and the distal side member are in contact with each other to form a boundary surface, the boundary surface being inclined relative to a vertical plane of a central axis of the optical fiber,
• at least a part of the boundary surface that is irradiated with the light from the optical fiber is a flat surface inclined relative to the vertical plane, and
• an inclination angle θb of the boundary surface satisfies the following expression:
• 0<θb≤π/2+[arcsin(NA/n1)]−[arcsin(n2/n1)]
• where NA is a numerical aperture of the optical fiber, n1 is a refractive index of the proximal side member, n2 is a refractive index of the distal side member, and θb is an acute angle formed between the central axis of the optical fiber and the boundary surface in a longitudinal section of the optical path changing member.