An optical axis adjustment device for a headlamp unit to be mounted in a vehicle is configured to adjust an optical axis of the headlamp unit. The optical axis adjustment device includes an engine temperature acquirer, a displacement amount calculator, and an optical axis adjuster. The engine temperature acquirer is configured to acquire a temperature in an engine compartment of the vehicle. The displacement amount calculator is configured to calculate a displacement amount of the optical axis on a basis of the acquired temperature. The optical axis adjuster is configured to adjust the optical axis of the headlamp unit on a basis of the calculated displacement amount.