A power transmission device (1) comprises a drive roller (2) and a driven roller (3) which are made of a magnetic material, rotatable about axes (X1, X2) different from each other, and provided with agap (15) therebetween, a case (4) which encloses the rollers (2, 3), an input shaft (16) and an output shaft (17) which are connected coaxially with the rollers (2, 3), respectively, through the case(4), and a magnetic fluid (18) contained in the case (4) so as to fill the gap (15). An electromagnetic coil (21) which generates magnetism and a magnetic path (24) of the magnetism generated by theelectromagnetic coil (21) such that a magnetic field (MF) from the drive roller (2) or the driven roller (3) to the driven roller (3) or the drive roller (2) via the gap (15) is generated are formed.