A walker is provided. The walker includes a walker body, a handle unit, a first wheel unit and a second wheel unit. The handle unit is connected to the walker body. The first wheel unit is connected to the walker body. The second wheel unit is connected to the walker body. In an auxiliary mode, a first gap is formed between the first wheel unit and the second wheel unit. In a driving mode, a second gap is formed between the first wheel unit and the second wheel unit. The first gap is smaller than the second gap.