Single crystal samples of near-stoichiometric Ni2MnGa alloy with orthorhombic seven-layered martensitic structure have been investigated. Measured lattice parameters are a=0.619 nm, b=0.580 mn and c=0.553 nm (in coordinates of parent cubic phase) at ambient temperature. Giant magnetic-field-induced strain (MSM effect) of about 10% was observed in this phase. Microstructure change under applied magnetic field was studied by means of X-ray diffraction and optical microscopy methods. It was shown that the mechanism of MSM effect in sevenlayered orthorhombic martensitic phase is similar to the mechanism found before in five-layered tetragonal martensite. Giant magnetic-field-induced strain in orthorhombic phase is caused by sample transformation from one single-variant state (a axis parallel to the field) by twinning ((101) [ 10bar 1] system) to another single-variant state (c axis parallel to the field).