A plasmonic infrared (IR) filter was experimentally and theoretically investigated. A localized surface plasmon polariton (LSPP) mode which was angle-independent in almost fully incident angle was observed. Through the use of the LSPP mode, an IR reflection-type notch filter with an ultrahigh immunity for the angular deviation was realized. An angle-independent reflection dip was designed at λ = 6.2 μm with a full-width at half-maximum of 0.5 μm. The experimental result shows that the position and the line shape of the resonant dip at λ = 6.2 μm remain the same for an increasing incident angle from 20° to 60°. The optical properties can be engineered by tuning thickness of the cavity layer. The proposed notch filter presents a large angular tolerance means that the superior angular stability makes it more feasible as it is put into practical applications.