A model in which a baryon can be regarded as a bound state of two particles has been generalized to be approximately invariant under SU(6). In the model, one of the constituent particles of a baryon can be regarded as a quark and the other particle can be considered as a tightly bound state of two quarks, or diquark. The quark is taken to belong to a six-dimensional representation of SU(6), while the diquark is taken to belong to a twenty-one-dimensional representation. With this model, which can be considered as a specific dynamical approximation to the three-quark model, the baryon medium-strong mass splittings are calculated beyond lowest-order perturbation theory. The model provides a mechanism for breaking the Gell-Mann-Okubo baryon-octet mass formula while breaking the baryon-decuplet equal-spacing rule by a smaller amount.