Analysis of NH3 gas under various humidity conditions was conducted using a waveguide surface plasmon resonance (SPR) sensor with dual sensing parts. Two pairs of Ag films/sensing polymer films were prepared separately on a waveguide core of BK-7 slide glass. Poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) were used as sensing materials. A white light was guided through the core by illuminating the substrate edge, and the SPR property was investigated by observing the output light spectrum. The thicknesses of PAA and PVA films were adjusted to induce SPR at different wavelengths. PAA exhibited remarkable response against NH3 gas, but it also exhibited a strong dependence on humidity. In contrast, PVA responded to humidity but hardly responded to NH3 gas below 20 ppm. The dual sensing would allow us to conduct precise NH3 measurements under various humidity conditions.