In the continuous casting of steel, the state of the mold flux plays a significant role in the quality of the steel as well as the casting operation. In this study, the state of the mold flux was estimated by measuring the friction force of mold oscillation and establishing a lubrication model. The friction force was measured with a slab caster equipped with hydraulic oscillators, and the relationship between the friction force and continuous casting factors such as the mold oscillation conditions, mold flux properties and casting speed was investigated. The main results are summarized as follows.