A body fluid compatible and biocompatible resin for use in a medical treatment involving a contact of said resin with at least one member selected from the group consisting of a body fluid and a biological tissue, which comprises at least one substituted oxyalkylene polymer having a weight average molecular weight of from 1,000 to 1,000,000 and represented by the following formula (1): wherein each of R1, R2 and R3 independently represents a hydrogen atom or a —CH2R4 group, and each R4 independently represents a hydroxyl group or a —OR5 group (wherein R5 represents a group selected from the group consisting of a C1-C10 aliphatic hydrocarbyl group, a C6-C10 aryl group, a —R6COOH group and a derivative thereof, and a —CH2—O—CH2—CH(OH)—CH2—OR7 group, wherein R6 represents a C1-C10 aliphatic hydrocarbylene group and R7 represents a C1-C10 aliphatic hydrocarbyl group or a C6-C10 aryl group), provided that all of R1, R2, and R3 are not simultaneously hydrogen atoms; and 10≦x≦10,000 and 0≦y≦10,000.