We use external magnetic field to probe the detection mechanism of superconducting nanowire single photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter $Delta$ across the whole width of the superconducting nanowire after absorption of the single photon) does not explain observed weak field dependence of the photon count rate (PCR) for photons with $lambda$=450 nm and noticeable {it decrease} of PCR (with increasing the magnetic field) in some range of the currents for photons with wavelengths $lambda$ =450-1200 nm. Found experimental results for all studied wavelengths $lambda = 450-1550$ nm could be explained by the vortex hot spot model (which assumes partial suppression of $Delta$ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed $Delta$.
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