  The Ising chain consisting of the antiferromagneticaly coupled ferromagnetic
trimer is considered in the external magnetic field. In the framework of the
transfer-matrix formalism the thermodynamics of the system is described. The
magnetization per site (m) is obtained as the explicit function of the external
magnetic field (H). The corresponding plots of m(H) are drawn. Two
qualitatively different regions of the values of coupling constants are
established: weak antiferromagnetic coupling (J_A<3J_F) and the strong
antiferromagnetic coupling $(J_A\geq3J_F)$. For the latter case the
magnetization curve with plateau at $m/m_{sat}=1/3$ is obtained. It is proven
that the plateau is caused by the stability of spatially modulated spin
structure <3111>. The values of magnetic field determining the width of the
plateau are obtained in the limit of zero temperature.
