The recently discovered kagome superconductors AV 3 Sb 5 (A = K, Rb, Cs) [1-3] possess a unique band structure with van Hove singularities and Dirac dispersions [4,5], in which unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking have been reported [6][7][8][9][10][11]. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries, including exotic states with chiral and topological properties accompanied by a sign-changing superconducting gap [12][13][14][15][16]. Experimentally, however, the phase information on the superconducting gap in AV 3 Sb 5 is still lacking. Here we report the electron irradiation effects in CsV 3 Sb 5 using introduced impurities as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, a highly anisotropic fully-gapped state changes gradually to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under high pressure show that the double superconducting dome in the pressure-temperature phase diagram survives against sufficient impurities. These results are strong bulk evidence that CsV 3 Sb 5 is a nonchiral, anisotropic s-wave superconductor with no sign change both at ambient and high pressure, which provides a clue to understanding the relationship between CDW and superconductivity in kagome superconductors.