Monolayers comprised of banana-shaped achiral molecules are modeled taking into account the hard-core repulsive force and electrostatic interaction between constituent molecules and the water surface. The model predicts the achiral-chiral transition of orientational structure along with in-plane nematic ordering during a monolayer compression due to the 2D geometry of monolayers. This model further reveals that in-plane nematic order is established beforehand of the chiral transition since banana-shaped molecules behave as rod-shaped molecules with variable aspect ratio in 2D geometry. It indicates the importance of in-plane nematic order in the achiral-chiral phase transition of monolayers comprised of banana-shaped achiral molecules.