One of the most important materials for blown film is high-density polyethylene (HDPE) with wide molecular weight distribution. First, we computed a wall stress at the entrance of a spiral groove in a particular die during blown film processing on a particular condition, to which a similar condition is widely utilized in a film works. The computed value is about 170 kPa, while the HDPE melt slips at die wall at stresses above approximately 50 kPa. The stress of 170 kPa is sufficiently large for the slip occurrence of the melt. Then, we investigated the effects of wall slip and melt viscosity on film thickness distribution in the circumferential direction; the distribution tends to decrease with decreasing wall slip and melt viscosity. This tendency is explained by considering flow distribution in a spiral mandrel die and polymer melt flow characteristics.