A polyimide foam formed of an aromatic polyimide prepared from a 3,3′,4,4′-biphenyltetracarboxylic acid component and an aromatic diamine component. The polyimide foam has flexibility at least such that no cracking occurs when a specimen of the polyimide foam with a 1 cm by 1 cm cross-section and a length of 5 cm is deformed until both ends thereof come into contact with each other to make a closed loop. The polyimide foam is obtained by dissolving a 3,3′,4,4′-biphenyltetracarboxylic acid component and an aromatic diamine component in a solvent in the presence of an acid phosphoric ester having a specific structure to prepare a polyimide precursor and heating the polyimide precursor to expand.