To provide a collapsible mount type lens barrel whose housing length is shortened further, and a collapsing method.

In the collapsible mount type lens barrel having a plurality of lens groups positioned on a single optical axis in a photographing feasible state, a plurality of lens groups include at least one lens group positioned separately from the single optical axis in a photographing unfeasible state, and a distance between the lens group positioned nearest to an object side and the lens group positioned nearest to an image side in the photographing feasible state is different from a distance between the lens group positioned nearest to the object side and the lens group positioned nearest to the image side in the photographing unfeasible state.