Biomolecules commonly show drastic differences in biochemical properties according to their chirality. Separation of enantiomers thus has great industrial importance. With recent developments of microfluidics, chiral resolution by flow has received considerable attention and demonstrated in a number of numerical and experimental studies. In this work, we theoretically clarify the conditions required for flow fields to induce the chiral separation. We study the separation of chiral objects with an arbitrary and rigid shape induced by a linear flow field at low Reynolds numbers. Based on a parity symmetry argument, we show that the rate-of-strain field is essential for inducing chirality-dependent drift motion. We further consider the separation of a swarm of enantiomers and derive that the flow field should be quasi-two-dimensional for efficient resolution. The results are demonstrated by Langevin dynamics simulations incorporating hydrodynamic interactions.