In an inverted tandem organic solar cell with a bulk heterojunction (BHJ) unit cell, strange s-shape appears in the current-voltage characteristics. We prepared a tandem cell composed of a BHJ bottomunit cell and a p-n stack type top unit cell with an interlayer in between the unit cells. Since the BHJ active layer has symmetric structure, an electron blocking layer is indispensable in the interlayer. The photovoltaic characteristics can be reproduced by an equivalent circuit with an inverted diode between two unit cells. The s-shape can be understood in terms of the reverse rectification by the electron blocking layer.