The observations of the slow shocks associated with the interplanetary coronal mass ejections near 1 AU have seldom been reported in the past several decades. In this paper we report the identification of an interplanetary slow shock observed by Wind on September 18, 1997. This slow shock is found to be just the front boundary of a magnetic cloud boundary layer. A self-consistent method based on the entire R-H relations is introduced to determine the shock normal. It is found that the observations of the jump conditions across the shock are in good agreement with the R-H solutions. The intermediate Mach number MI = Un/(VAcosθBn) is less than 1 on both sides of the shock. In the upstream region, the slow Mach number Ms1 = Un1/Vs1 is 1.44 (above unity), and in the downstream region, the slow Mach number Ms2 = Un2/Vs2 is 0.8 (below unity). Here Vs and VA represent the slow magnetoacoustic speed and Alfvén speed respectively. In addition, the typical interior magnetic structure inside the shock layer is also analyzed using the 3s time resolution magnetic field data since the time for the spacecraft traversing the shock layer is much longer (about 17s). As a potential explanation to the formation of this kind of slow shock associated with magnetic clouds, this slow shock could be a signature of reconnection that probably occurs inside the magnetic cloud boundary layer.