TECHNOLOGY FOCUS - POLYMERS - Preparation (claimed): An aliphatic group is introduced into the main chain structure by aromatic nucleophilic substitution using dithiol compound of formula (11): HS-R3-SH, to obtain the sulfonic acid group-containing polymer.R3=2-20C alkylene group or aralkylene group containing sulfur or oxygen.Preferred Polymer: The polymer further has structural units of formulae (3-6).X,Y'=same as defined above;R1=same as defined above, preferably direct bond;Z1,Z3=oxygen or sulfur, preferably oxygen;Z2=oxygen, sulfur, -C(CH3)2-, -C(CF3)2-, -CH2-, cyclohexylene or direct bond, preferably direct bond;Ar2=trivalent or tetravalent group containing aromatic group, preferably benzene ring;q=1 or 2, preferably 1;Ar1=same as defined above, preferably groups of formulae (7-10);ando,p,r,s=1-1000.(n+o+r)/(n+m+o+p+r+s)=0.05-0.70, preferably 0.15-0.70, (o+p)/(n+m+o+p+r+s)=0.01-0.99, preferably 0.01-0.70, (r+s)/(n+m+o+p+r+s)=0-0.10 and (n+m)/(n+m+o+p+r+s)=0.01-1, preferably 0.05-0.8. Preferred Definitions:    R2=linear alkylene group.    EXAMPLE - Sodium salt of 3,3'-disulfonic acid-4,4'-dichlorodiphenylsulfone (in g) (25), 2,6-dichlorobenzonitrile (64.19), 1,6-hexanedithiol (63.74), molecular sieve 3-A (60), potassium carbonate (64.47) and N-methyl-2-pyrrolidone (NMP) were stirred at 150degrees C for 30 minutes. The temperature was raised to 215degrees C and the contents were reacted for 12 hours. The molecular sieve was filtered and the polymerized solution was settled. The polymer was washed with water and dried at 110degrees C to obtain sulfonic acid group-containing polymer. The obtained polymer (7) was dissolved in NMP (28), cast on a glass plate and heated at 80degrees C, 150degrees C and 120degrees C. The formed film was peeled and immersed in sulfuric acid aqueous solution (2 mol/l) for 1 hour. The film was removed and washed with water, and a polymer electrolyte membrane was produced. The polymer electrolyte membrane had high ion exchange capacity and provided direct methanol type fuel cells with high output characteristics and excellent handleability.