The paper describes a technique to visualize subbands in semiconductor superlattices, which consists of a photon counting method to measure extremely weak photoluminescence (PL) and an image processing using a personal computer for visualization. The technique has been applied to measure the electric-field dependence of the subbands for GaAs/AlAs superlattices with layer thicknesses of 6.8 nm/1.7 nm (sample 1), 6.2 nm/3.4 nm (sample 2) and 6.0 nm/5.0 nm (sample 3), respectively. The field effects of the subbands such as the resonance and Stark-ladder transition are clearly visualized from the PL images. We observed the PL emission between the second Γ(Γ₂) and heavy hole (hh₁) states due to the resonance between the ground Γ(Γ₁) and Γ₂ states, and the PL emission between the ground X (X₁) and hh₁ states due to the resonance between X₁ and Γ₂ states. Furthermore, we observed the PLs from the Stark-ladder transitions between the Γ₁(+1) and hh₁ states and between the X₁(+1/2) and hh₁ states. This technique is useful for studying electronic states in semiconductor superlattices.