Provided are a lens unit and a vehicle-mounted infrared lens unit capable of correcting a focal shift due to a temperature change without increasing the apparatus size, complicating a production process, and increasing the costs. In a vehicle-mounted infrared lens unit 4 configured such that a plurality of infrared lenses are held by a barrel 30 and a spacer 40 is interposed between two infrared lenses, a first infrared lens 10 is held sandwiched between the spacer 40 and a lens holding portion 31 of the barrel 30. The spacer 40 and the barrel 30 are formed of materials having different thermal expansion coefficients. An O-ring 60 is interposed between a lock portion 32 of the lens holding portion 31 and the first infrared lens 10. Thermal expansion of the spacer 40 allows the first infrared lens 10 to move in the axial direction against the elastic force of the O-ring 60. When the spacer 40 is shrunken, the elastic force of the O-ring 60 allows the first infrared lens 10 to move in the opposite direction.