What is provides is a brain activity deducing apparatus, a brain activity deducing method, a brain activity measuring apparatus, a brain activity measuring method, and a brain-machine interface device, capable of deducing a brain activity signal of a source subject. The information presentation device presents perceptible information to the first subject. The brain activity measurement device acquires a brain activity signal representing a brain activity of the first subject. The individual conversion device deduces a brain activity signal of the second subject from the acquired brain activity signal based on the individual conversion information, which correlates the brain activity signal of the first subject and the brain activity signal of the second subject.