Abstract: A text input section (1) divides an inputted text into sentences and attaches a number to each of the sentences, which is stored in a text data base together with the number. An important word recognizing section (2) generates a list of important words for each sentence to store it in a storing section (8). An important word weighting section (3) weights each important word. A relation degree computing section (4) computes a relation degree between an attention sentence and a precedent sentence. An important degree computing section (5) computes an importance degree of each attention sentence. A tree structure determining section (6) determines a parent sentence of the attention sentence and determines a tree structure of the inputted text.