We have succeeded in growing a phase in CaAlSi with Al and Si atoms being distributed regularly in the AlSi layer corresponding to the B2 plane in the AlB2 -type structure without a superlattice along the c axis. The field-induced magnetic response in a phase exhibits an almost isotropic characteristic for the respective crystal axis. Moreover, the 2Δ(0) kB Tc and Δ Cel γN Tc estimated by heat capacity measurements are ∼3.5 and ∼1.4, respectively, indicating a BCS superconductor with a weak coupling. Although CaAlSi has a two-dimensional layered structure, these results strongly suggest that the superconductivity of the phase in CaAlSi is derived from a three-dimensional electronic ground state such as a conventional s -wave superconductor, which is markedly different from those of superstructured CaAlSi. © 2006 The American Physical Society.