To provide a joining structure and method for securely joining a lens and a lens holder without using an adhesive.

At least three projections 12 are formed in a collar section 11 of an objective lens 10. A lens loading surface 23 of the lens holder 20 is provided with recesses 24 corresponding to projections of the objective lens. The objective lens is fitted into a stepped section 22 of the lens holder in such a manner that the projections of the objective lens exist in the recesses of the lens holder and while the horn of an ultrasonic welding machine is pressurized by pressing the horn to the rear surface side of the projections of the collar section, in this state, the horn is ultrasonically vibrated. As a result, the projections are melted and the objective lens is joined to the lens holder.