A communication device includes a storage unit to store quotients and remainders associated with multiplication values obtained by multiplying a specified integer number, which is expressed in a form of (2β+α) where β is a positive integer number and α is a positive integer number other than integral multiples of 2, respectively, the quotients and the remainders being obtained by dividing the multiplication values by 2β, respectively, a first unit to divide a dividend by 2βand calculate a quotient and a remainder, a second unit to obtain a quotient, which corresponds to the remainder from the storage unit, and a third unit to determine that the data length of the packet data is normal, when a combination of the quotient and the remainder calculated by the first unit is in the storage unit.