We propose using a pair of bumps bordering the conventional trench-surrounded metal nano slit in order to confine the surface waves and further enhance the slit transmission. The bump height of 1μm is larger than the depth of penetration on air side of the surface waves. The reflectivity of such bumps is larger than 95%. A very large slit transmission, which is 50% of the energy of the incident beam impinging on the entire size 13μm of the trench-surrounded slit structure, is obtained through the metallic slit of 50nm width and 400nm depth. The bumps enhance the transmission by 1.75 fold.