To provide a device for treating fluid with magnetism which is effective for the prevention of scale, the strengthening of concrete, the reforming of liquid fuel, the prevention of putrefaction of alcohol beverage or water- containing beverage and the acceleration of aging.

A large diameter ring-like permanent magnet is housed in a pipe line through which a fluid flows, a small diameter ring like magnet is engaged in the large diameter magnet ring, a flow passage is formed in a gap between the large diameter magnet and the small diameter magnet, a closed circuit of the line of magnetic force is formed between the large diameter magnet and the small diameter magnet and the line of magnetic force crosses the flowing fluid at a right angle.