  We measured the magnetic penetration depth $\lambda(T)$ in single crystals of
LaOs$_{4}$Sb$_{12}$ ($T_c$=0.74 K) down to 85 mK using a tunnel diode
oscillator technique. The observed low-temperature exponential dependence
indicates a s-wave gap. Fitting the low temperature data to BCS s-wave
expression gives the zero temperature gap value $\Delta (0)= (1.34 \pm 0.07)
k_B T_c$ which is significantly smaller than the BCS value of 1.76$k_B T_c$. In
addition, the normalized superfluid density $\rho(T)$ shows an unusually long
suppression near $T_c$, and are best fit by a two-band s-wave model.
