A magnetooptical device which defines a Faraday rotation angle theta expressed by 44 deg.<=theta<=46 deg. when light having a wavelength lambd (1570 nm<=lambd<=1620 nm) impinges thereupon. A magnetic garnet material expressed by a general formula: BiaM13-a Fe5-bM2bO12 is used. M1 is at least one kind of element that is selected from among Y, La, Eu, Gd, Ho, Yb, Lu and Pb; M2 is at least one kind of element that is selected from among Ga, Al, Ti, Ge, Si and Pt; and a and b satisfy 1.0<=a<=1.5 and 0<=b<=0.5, respectively.