A magnesium boride thin film having a B-rich composition represented by the general formula of MgBx (x=1 to 10) and a superconducting transition temperature of 10K or more has superior crystallinity and orientation and is used as a superconducting material. This thin film is formed by maintaining a film forming environment in a high vacuum atmosphere of 4×10−5 Pa or less, and simultaneously depositing Mg and B on a substrate maintained at a temperature of 200° C. or less so as to grow the film at a growth rate of 0.05 nm/sec or less. It is preferable to supply an Mg vapor and a B vapor into the film forming environment at an Mg/B molar ratio of 1/1 to 12/1.