Rotation of a reflector leads to a phenomenon of self-mixing interference (SMI) of a beam of unfocused laser. There are methods proposed for a measurement of angle by using the unfocused-laser SMI. However, the range of angle measurement is limited by the size of the utilized laser aperture. The SMI of the beam of focused laser is studied. A new approach to measure an angle by use of focus-laser SMI is proposed. The approach is able to achieve an angle measurement with an order of magnitude higher than the existent methods do. The increase of range in angle measurement by the proposed approach is achieved by placing a focusing lens into the optical path. The lens are used to collect the light, which deviates from emergent one, back into the resonator of laser. The SMI occurrs as a consequence of the collected light. The proposed approach is experimentally validated. The experiments also show that the increase in the focal length of the utilized lens can enlarge the range of angle measurement.