The fluidity of pure aluminum and Al-Si alloys was investigated for casting thin products using a spiral die in die casting. An aluminum alloy with good fluidity can be die-cast into thin products. For a Si content of less than 6 mass%, the fluidity increased with decreasing Si content. For a Si content of greater than 6 mass%, the fluidity increased with increasing Si content. The fluidity was affected by latent heat, flowability in the semisolid state, and heat transfer between the die and metal. For pure aluminum, the latent heat is small and there is no semisolid state. However, pure aluminum has excellent fluidity because the heat transfer between the die and metal is small. For Al-25%Si, the latent heat is very large and flowability increases in the semisolid state. Therefore, the fluidity of Al-25%Si is high. Fluidity typically increases with increasing die temperature. The increase in fluidity due to an increase in die temperature for the pure aluminum is small compared with that for hypoeutectic Al-Si alloys. This means that the heat transfer between the pure aluminum and the die is smaller than that for hypoeutectic Al-Si alloys. Therefore, the influence of die temperature on the fluidity of the pure aluminum is small. It is estimated that the chill layer of the pure aluminum rapidly peels from the die, decreasing the heat transfer between the pure aluminum and the die.