PURPOSE: To ensure the discrimination of encoded data by constituting a run length control code after conversion with maintaining a 2-7 encoding form, and with decreasing the maximum/minimum ratio of the density of one binary digit value to the specific ratio.

CONSTITUTION: By inputting a present condition X18 inputted from original data 14 and a resister 17, a sequence condition/output logical circuit 16 of an encoder 12 converts input data into encoded data 15 and a secondary condition X'19, output them, and the conversion is executed by writing, into a ROM in a logical circuit 16, the encoded data and the secondary condition X' corresponding to an address using the original data 14 and the condition X as the address. When the data after the encoding by means of the encoder 12 are divided into encoded blocks at prescribed lengths, and the arbitrary continuous three encoded blocks are obtained, the ratio of the maximum and the minimum of the number of '1's included in the encoded blocks can be restricted at 2:1, and the discrimination of the encoded data can be ensured.