Nonlinear characteristics induced by accumulation of photogenerated carriers in a cap layer on a GaAs/GaAlAs multi‐quantum well (MQW) structure can be observed at 300 K under weak photoexcitation (2.5 mW/cm2). Photoinduced currents (PIC) are measured for various structures of cap layer. Negative differential properties are observed in the relations between PIC and forward bias voltage for the MQW structures with a cap layer including InAs/GaAs short period superlattice. The phenomenon is reduced in the case of a cap layer including an InGaAs alloy and disappears in the case of that composed of only GaAs. These experimental results are explained reasonably considering the capability of the cap layer to accumulate photogenerated carriers.