A two-dimensional code authenticating device reads a self-authentication two-dimensional code to obtain an RS bit string, and detects a bit string c′ as an error using the RS bit string. Next, an exclusive OR between a bit string l included in the RS bit string and the bit string c′ detected as an error is calculated to obtain a bit string c′, and a bit string md is obtained by decrypting the bit string c′ through a scheme corresponding to the encryption. Subsequently, it is determined whether or not the decrypted bit string md matches the bit string m included in the RS bit string, thereby authenticating the self-authentication two-dimensional code.