We present the detection of a large population of ultradiffuse galaxies (UDGs) in two massive galaxy clusters, Abell S1063 at z = 0.348 and Abell 2744 at z = 0.308, based on F814W and F105W images in the Hubble Frontier Fields Program. We find 47 and 40 UDGs in Abell S1063 and Abell 2744, respectively. Color-magnitude diagrams of the UDGs show that they are mostly located at the faint end of the red sequence. From the comparison with simple stellar population models, we estimate their stellar mass to range from 108 to 109 M . Radial number density profiles of the UDGs show a turnover or a flattening in the central region at r < 100 kpc. We estimate the total masses of the UDGs using the galaxy scaling relations. A majority of the UDGs have total masses M200 = 1010-1011 M , and only a few of them have total masses M200 = 1011-1012 M . The total number of UDGs within the virial radius is estimated to be N(UDG) = 770 114 for Abell S1063 and N(UDG) = 814 122 for Abell 2744. Combining these results with data in the literature, we fit the relation between the total numbers of UDGs and the masses of their host systems for M200 > 1013 M with a power law, N(UDG) = . These results suggest that a majority of the UDGs have a dwarf galaxy origin, while only a small number of the UDGs are massive L* galaxies that failed to form a normal population of stars.