A method for developing a latent image formed on an image carrier with a developing liquid includes storing a liquid consisting of a developing substance and a carrier liquid developing and sensing the liquid fed from the liquid storing section thereon and sensing a liquid level in the liquid storing section using a liquid level sensor. The liquid level sensor includes a magnetic force generating member for generating a magnetic force, a magnetic force sensing device for sensing the magnetic force, and floats movable up and down in accordance with the liquid level in the liquid storing section. A distance between the magnetic force generating member and the magnetic force sensing device varies in accordance with the movement of the floats, allowing the liquid level to be determined on the basis of the output of the magnetic force sensing device. An image forming method including a developing device is also disclosed.