  The excitation energies of the $\Lambda_{c}$ and $\Lambda_{b}$ baryons are
investigated in a finite-size diquark potential model, in which the heavy
baryons are treated as bound states of a charm quark and a scalar-isoscalar
diquark. The diquark is considered as a sizable object. The quark-diquark
interaction is calculated as a sum of the quark-quark interaction which is
assumed to be half of the quark-antiquark interaction for the color singlet.
The potential parameters in the quark-antiquark interaction are fixed so as to
reproduce the charmonium spectrum. We find the diquark size to be 1.1 fm for
the diquark mass 0.5 GeV/c$^{2}$ to reproduce the $1p$ excitation energy of
$\Lambda_{c}$. In this model, the $\Lambda_{c}$ and $\Lambda_{b}$ excitation
spectra are reproduced well, while this model does not explain
$\Lambda_{c}(2765)$, whose isospin nor spin-parity are unknown yet. Thus, the
detailed properties of $\Lambda_{c}(2765)$ is very important to the presence of
the diquark in heavy baryons as a effective constituent. We also discuss the
$\Xi_{c}$ spectrum with the scalar strange diquark.
