This invention provides a binder for fuel cell, which has high adhesion, low methanol solubility, high methanol permeability and high proton conductivity, a composition for electrode formation, an electrode for a fuel cell, and a fuel cell using them. The binder is particularly suitable as a binder for a direct methanol-type fuel cell which requires high proton conductivity. The binder for a fuelcell comprises a block copolymer that comprises blocks comprising repeating structural units of a protonic acid group-containing divalent aromatic group and blocks comprising repeating structural units of a protonic acid group-free divalent aromatic group and has a glass transition temperature (Tg) of 180 DEG C or below. In particular, preferably, the block copolymer has an ion exchange group equivalent of 200 to 1000 g/mol and a weight retention of not less than 90% as measured by immersion in a 64 wt% aqueous methanol solution at 25 DEG C for 24 hr.