A method of properly correcting a base line. A concept of flexibility of a base line is introduced as an input index and a gravitation is presumed between the base line and signal data which is measured, thereby constructing a method of correcting the base line which changes gentler than a change in signal with respect to time in a peak area and which is sensitive to an area where a value of the signal data is small. A base line like a natural and smooth curve which isn't easily influenced by a local change in signal such as noise or the like can be set by an input of the flexibility.