To provide a manufacturing method of a lens assembly capable of improving the airtightness and durability of the joint between the lens and the frame while simplifying and reducing the cost, and to provide a lens assembly and an endoscope.

The manufacturing method of a lens assembly of this invention employs a thermosetting resin adhesive including a filler between the top material 9 and the plano-convex lens 13, and hardens the above thermosetting resin adhesive by heating to connect the top material 9 and the plano-convex lens 13. The obtained lens assembly has the top material 9 and the plano-convex lens 13 connected together with an adhesive layer 1129E composed of a thermosetting resin adhesive including a filler.