Beamer math mode font is different

Question

I have the following beamer layout. This layout is great but the problem is that when I type math equations, the font is not set to the default font as in the picture. I think some command in the below code makes the font different but I don't know what is that. Please give a way to set math font as default.

\documentclass[11pt]{beamer}
\hypersetup{pdfencoding=auto}

\usetheme{Berkeley}

\usepackage{xcolor}
\usepackage{amssymb,amsmath,graphicx}
\usepackage{kotex}
\usepackage{multimedia}
\usepackage{setspace}
\usepackage{multicol}
\usepackage{verbatim} %use 

%% Color Setting %%
\definecolor{Grey}{rgb}{0.6,0.6,0.6}
\definecolor{GSHSred}{RGB}{105,0,0}
\definecolor{GSHSRED}{RGB}{80,0,0}
\definecolor{gshsred}{RGB}{240,210,210}
\setbeamercolor{title}{bg=white,fg=GSHSred}
\setbeamercolor{author}{fg=GSHSred}
\setbeamercolor{institute}{fg=GSHSred}
\setbeamercolor{date}{fg=GSHSred}
\setbeamercolor{logo}{bg=GSHSred}
\setbeamercolor{sidebar}{bg=GSHSred}
\setbeamercolor{frametitle}{bg=white,fg=GSHSred}
\setbeamercolor{section in sidebar}{use=sidebar,bg=white,fg=sidebar.bg}
\setbeamercolor{subsection in sidebar}{parent=section in sidebar}
\setbeamercolor{subsubsection in sidebar}{parent=subsection in sidebar}
\setbeamercolor{item}{fg=GSHSred}
\setbeamercolor{block title}{bg=GSHSRED,fg=white}
\setbeamercolor{block body}{bg=gshsred,fg=black}

%% Frametitle Setting %%
%\setbeamerfont{frametitle}{series=\bfseries}
\setbeamertemplate{frametitle}
{ \vspace*{-10mm}
  \leavevmode
  \hspace*{3pt}
  \begin{beamercolorbox}[wd=\paperwidth,ht=1ex,dp=1ex]{frametitle}
    \hspace*{7pt}\underline{\makebox[0.6\paperwidth][l]{
    \Large{\insertframetitle}}}
  \end{beamercolorbox}
}


%% Title Page Setting %%
\setbeamerfont{title}{series=\bfseries}
\setbeamertemplate{title page}{
  %% Background Logo
  \begin{picture}(0,0)%
    \setlength{\unitlength}{1cm}% default
    \protect\put(0,0){%
    \begin{picture}(6,6)(4,10)%
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      %\includegraphics[width=0.4\paperwidth]{./logo/100_01.png}%
      \includegraphics[width=0.7\paperwidth]{./logo/gshslogo2.jpg}%
    \end{picture}%
    }%
  \end{picture}%
  \vfill
  \vspace*{10mm}
  \raggedleft
  %% Title
  \usebeamerfont{title}\usebeamercolor[fg]{title}\inserttitle\par
  \vskip 2mm
  %% Subtitle
  \ifx\insertsubtitle\@empty
  \else\usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\insertsubtitle
  \fi

  \vskip 3mm
  %% Horizontal line
  \usebeamercolor[fg]{title}\hrule height 2pt\hfill
  \vskip 10mm
  %% Author
  \usebeamercolor[fg]{author}\usebeamerfont{author}\insertauthor
  \vskip 1cm
  %% Institute
  \usebeamercolor[fg]{institute}\usebeamerfont{institute}\insertinstitute
  \vskip 1cm
  %% Date
  %\usebeamercolor[fg]{date}\usebeamerfont{date}\insertdate
  \vfill
}

%% itemize bullet setting %%
\setbeamertemplate{itemize items}[ball]

%% block setting %%
\setbeamertemplate{blocks}[rounded][shadow=true]