Hi there Byung Hwa,
I’m sorry it’s not performed as expected when viewed on handheld devices.
Interestingly, given a default theme, the ticket form seems to do a much better job than in the case of your own website. It also looks like the ticket form has been customized somewhat via CSS added by your theme (Eventica WP).
Unless these are tweaks and adjustments you made yourself, it may be worth approaching the theme vendor about this A) because they can fix these rules and that will benefit other users of the theme and B) because they are best positioned to adjust things so that they stay in line with the general aesthetic your theme is aiming for.
Could you send them a note and see if they can help out with this one?