LM555: LM555 sometimes dead

Yoon,

My suspicion is inductive kickback from the heating element. Although it is a resistive element, they are generally made of wound coils which also look like an inductor. In this case adding a transient voltage suppression diode (TVS diode) to the supply line could help with this scenario.  

We have a series on EOS. If you look to the right of this link you will see 4 videos 12.1 through 12.4 that will walk you through these events and choosing components for protection.

https://training.ti.com/node/1138807

Dear Chris.

Thank you so much !

After checking 12.1~4. I applied some TVS to my circuit.

Best Regards,Yoon.