Chainer is a robust, adaptable, and user-friendly framework designed for building neural networks. It facilitates CUDA computation, allowing developers to utilize a GPU with just a few lines of code. Additionally, it effortlessly scales across multiple GPUs. Chainer accommodates a wide array of network architectures, including feed-forward networks, convolutional networks, recurrent networks, and recursive networks, as well as supporting per-batch designs. The framework permits forward computations to incorporate any Python control flow statements without compromising backpropagation capabilities, resulting in more intuitive and easier-to-debug code. It also features ChainerRLA, a library that encompasses several advanced deep reinforcement learning algorithms. Furthermore, with ChainerCVA, users gain access to a suite of tools specifically tailored for training and executing neural networks in computer vision applications. The ease of use and flexibility of Chainer makes it a valuable asset for both researchers and practitioners in the field. Additionally, its support for various devices enhances its versatility in handling complex computational tasks.