This study provides the result obtained by an optimum design method of smooth blasting in tunnelling hard rock. The proposed design method aims at minimize the total expense of the extra concrete cost in the region of overbreak and the labour cost required to remove the unbroken rock in the region of underbreak. The design variables are the look out angle θ and the collaring radius R. The constraint of look out angle is set in order to provide the workability of the drilling. The overbreak and underbreak distributions used in the present study are the testing results obtained from Shiwa tunnel in Sanyo Highway. The calculated results indicate that the look out angle needs only the minimum values which satisfy the workability of the drilling and that the optimum collaring radius depends on the drilling precision. The results also indicate that the improvement of drilling precision can save 7-10% of the total expense and the optimization of the design can save 6-11% of the expense.