To provide (i) a two-lens optical system, (ii) a beam combining module, (iii) a projector, and (iv) a method for assembling a two-lens optical system each of which allows for an improved lens positioning sensitivity [Means to Attain the Object] A two-lens optical system (100) includes a first lens (1) for use in collimation adjustment, the first lens being configured to move along only an optical axis of light emitted from a light source; and a second lens (2) for use in beam steering, the second lens being configured to move along only two axes perpendicular to the optical axis.