Maltose on treatment with dil. HCl gives D-glucose. Hydrolysis of maltose yields two moles of $$\alpha $$- D-glucose. Thus, it is composed of two $$\alpha $$-D-glucose units in which C-1 of one glucose unit (I) is linked to C-4 of another glucose unit (II). The free aldehyde group can be produced at C-1 of second glucose in solution and it shows reducing properties.
So, it is a reducing sugar.