Provided are a method and apparatus for generating a pseudo-random number which is unpredictable and which has a small memory work area, and also a method and apparatus for encrypting data, for each predetermined amount, based on the generated pseudo-random number. A seed is divided into a predetermined number of blocks, new blocks are created by calculating an exclusive-OR of the blocks being different from each other, and the new blocks are merged to generate a new pseudo-random number. The data is encrypted for each determined amount based on the generated pseudo-random number. At this time, a pseudo-random number to be used for the succeeding encryption is generated by using as a seed a predetermined amount of random number of the pseudo-random number used for the preceding encryption of the predetermined amount of data.