Abstract : The mass of the 1/2 + $uudd\bar s$ pentaquark is calculated within the framework of a semirelativistic effective Hamiltonian approach to QCD with instanton-induced forces, using a diquark picture. This approximation allows a correct treatment of the confinement, assumed here to be a Y-junction. With the [ud] diquark mass fitted on the $\Lambda$ -baryon, the ground-state pentaquark is found around 2.2 GeV.