Disclosed is a monofilament allowing contrast X-ray radiography. At least part of the monofilament is formed of a thermoplastic resin containing a radiopaque agent. The monofilament contains the radiopaque agent in the thermoplastic resin in a content of 30 to 80% by mass, and has a Youngs modulus of 0.1 to 5.0 cN/dtex and a fineness of 500 to 20000 dtex.