Toshio-san,
Theta JC(top) is 59C/W. However this number is only really useful if the plan is to attach a heatsink to the top of the device.
If the goal is the calculate the junction temperature based on the measured top of case temperature then Psi JT is needed.
Psi JT is 12.5C/W. Tj = Tc(Measured) + (Psi Jt)* Pd
Pd is device power dissipation.