The objectives of the present invention are: to find a biocompatible material which has excellent film formation properties and resistance to water dissolution and is able to be easily coated to various bases, while having excellent antithrombotic properties； and to provide an antithrombotic coating agent and an antithrombotic coating film, each of which uses this biocompatible material, and a medical device which is provided with this antithrombotic coating film. Means that have enabled the achievement of these objectives are: a copolymer which contains at least one repeating unit (A) represented by formula (1) and at least one repeating unit (B) represented by formula (2)； an antithrombotic coating agent which contains this copolymer and an organic solvent； an antithrombotic coating film which is formed of this copolymer； and a medical device which is provided with this coating film. (In formula (1), n represents an integer of 2-10； and R1 represents a methyl group or an ethyl group.) (In formula (2), R2 represents an aliphatic hydrocarbon group.).本發明發現具有優良之製膜性及耐水溶性，可容易地塗覆於各種基材，而且具有優良之抗血栓性的生體相容性之材料，而以提供利用其之抗血栓性塗佈劑及抗血栓被膜、以及具備該抗血栓被膜之醫療用具為課題。解決該課題之手段，為含有至少1種的下述式(1)(式中，n表示2~10之整數，R1表示甲基或乙基)表示之重複單位(A)、與至少1種的下述式(2)(式中，R2表示脂肪族烴基)表示之重複單位(B)的共聚物以及含有該共聚物與有機溶劑之抗血栓性塗佈劑、以該共聚物所形成之抗血栓性被膜及具備該被膜之醫療用具。
