  CaSb2 is a topological nodal-line semimetal that becomes superconducting
below 1.6 K, providing an ideal platform to investigate the interplay between
topologically nontrivial electronic bands and superconductivity. In this work,
we investigated the superconducting order parameter of CaSb2 by measuring its
magnetic penetration depth change {\Delta}{\lambda}(T) down to 0.07 K, using a
tunneling diode oscillator (TDO) based technique. Well inside the
superconducting state, {\Delta}{\lambda}(T) shows an exponential activated
behavior, and provides direct evidence for a nodeless superconducting gap. By
analyzing the temperature dependence of the superfluid density and the
electronic specific heat, we find both can be consistently described by a
two-gap s-wave model, in line with the presence of multiple Fermi surfaces
associated with distinct Sb sites in this compound. These results demonstrate
fully-gapped superconductivity in CaSb2 and constrain the allowed pairing
symmetry.
