In order to investigate the interaction between gas bubbles and solidified shell, in-situ observations using transparent substance have been made. The movement of argon gas bubbles has been observed and recorded by a VTR. The effect of diameter of gas bubbles, the angle of a copper plate and solid/liquid interfacial morphology on the entrapment of gas bubbles have been characterized. The entrapment of bubbles increases with decreasing the diameter of gas bubbles and with increasing the angle of the copper plate. Furthermore, it is found that the entrapment of gas bubbles increases with increasing the roughness of the solid/liquid interface. A physical model has been developed taking the force balance affecting a bubble into account. Using this model, the results of the present experiment have been consistently interpreted.