Signature is widely used to authorize who issued the document. However, signature has ambiguity, and it is difficult to distinguish the authentic signature from the mimicked signature only by bit-mapped patterns. On the other hand, altitude and direction of gripped pen under signing depends on the shape of writer's hand and the habit of writing. In this paper, we propose a new on-line writer verification method, which uses the pen movement in signing. From experimental results with 24 writers, verification rate of 100% is obtained.