A pre-chamber structure for a piston engine may include: a pre-chamber combustion chamber in which a mixture of fuel and air is combusted and a pre-chamber compression chamber including a sub-chamber communicating with the pre-chamber combustion chamber and having an inclined wall surface, and a main chamber extending from the sub-chamber, thereby preventing the fuel from being adsorbed and remaining in the wall surface of the pre-chamber compression chamber in a droplet state.