  The magnetic penetration depth ($\lambda$) as a function of applied magnetic
field and temperature in SrPt$_3$P($T_c\simeq8.4$ K) was studied by means of
muon-spin rotation ($\mu$SR). The dependence of $\lambda^{-2}$ on temperature
suggests the existence of a single $s-$wave energy gap with the
zero-temperature value $\Delta=1.58(2)$ meV. At the same time $\lambda$ was
found to be strongly field dependent which is the characteristic feature of the
nodal gap and/or multi-gap systems. The multi-gap nature of the
superconduicting state is further confirmed by observation of an upward
curvature of the upper critical field. This apparent contradiction would be
resolved with SrPt$_3$P being a two-band superconductor with equal gaps but
different coherence lengths within the two Fermi surface sheets.
