Network slicing is a key technology in 5G communications system, which aims to dynamically and efficiently allocate resources for diversified services with distinct requirements over a common underlying physical infrastructure. Therein, demandaware resource allocation is of significant importance to network slicing. In this paper, we consider a scenario that contains several slices in radio access networks with base stations sharing the same bandwidth. Deep reinforcement learning (DRL) is leveraged to solve this problem by regarding the varying service demands and the allocated bandwidth as the environment state and action, respectively. In order to tackle the annoying randomness and noise embedded in the received quality of experience (QoE) satisfaction ratio and spectrum efficiency (SE), we propose generative adversarial network (GAN) based deep distributional Q network (GAN-DDQN) to learn the distribution of action values. Furthermore, we estimate the distributions by approximating a full quantile function, so as to make the training error more controllable. For the sake of protecting the stability of GAN-DDQN's training process from the widely-spanning utility values, we also put forward a reward-clipping mechanism. Finally, we verify the performance of the proposed GAN-DDQN algorithm through extensive simulations.Index Terms-network slicing, deep reinforcement learning, distributional reinforcement learning, generative adversarial network, 5G