Mobile web page width is shrinked

I am using plugin of ‘Page Builder by SiteOrigin’ easily create multiple columns(blocks) on a page. e.g. put four text-editor blocks in horizontal direction and write codes or put images in the blocks. Looks fine on PC browser even if narrowing the browser until mobile’s pixcel width, but if I look the same page on iPhone or Android actual device, the page width shrinks to almost half of whole browser width. Tested it on i-max, i-craft, i-excel and i-transform. the result was same.
Should I look into the style.css and change attributes around @media ?
I really appreciate any advice or suggestions to fix it.

Regards,
Takeo