We have investigated electronic properties of self-assembled InAs quantum dots grown on GaAs surfaces by using metallic leads with narrow gaps. Although no intentional tunneling barriers were introduced, most of the conducting junctions showed single electron tunneling behaviors at low temperatures and exhibited clear shell fillings. The observed shell-dependent charging energies and tunneling conductances strongly suggest that the wave functions for higher shells are more extended in space. Furthermore, when coupling between electrons in the QDs and the electrodes was strong, enhancement in the linear conductance at zero bias was clearly observed when the number of electrons in QD was odd. This enhancement is attributed to Kondo effect and the estimated Kondo temperature T-K was 10-15 K. (c) 2006 Elsevier B.V. All rights reserved.