The cold dark matter (CDM) structure formation model predicts that about 5%-10% of a typical galactic halo of mass ∼ 1012 M⊙ is in substructures with masses ≲108 M⊙. To directly detect such substructures, we propose to observe dust continuum emission from a strongly lensed QSO host galaxy using a large submillimeter interferometer array with a high angular resolution of ∼0″.01, such as the planned Atacama Large Submillimeter Array (ALMA). To assess their observational feasibility, we numerically simulate millilensing of an extended circular source by a CDM substructure modeled as a tidally truncated singular isothermal sphere (SIS) embedded in a typical QSO-galaxy lens system, B1422+231, modeled as a singular isothermal ellipsoid (SIE) with an external constant shear and a constant convergence. Assuming an angular resolution of 0″.01, we find that the angular positions of ∼108 M⊙ substructures at several kpc from the center of the macrolens halo can be directly measured if the size of the dust continuum emission region and the gradient of the surface brightness at the position of the perturber are sufficiently large. From the astrometric shift on a scale of a few times 10 mas of an image perturbed by a subhalo with respect to an unperturbed macrolensed image, we can break the degeneracy between subhalo mass and distance provided that macrolensing parameters are determined from positions and fluxes of multiple images.