\documentclass{jarticle}
\usepackage{slashbox}
\usepackage{multirow}

\begin{document}

\begin{tabular}{|c|c|c|c|c|c|}
\hline
\multirow{3}*{\backslashbox{a\rule[-1.6em]{0pt}{1.8em}}{b\rule[-.5em]{0pt}{1.8em}}}& \multirow{3}*{c} &\multicolumn{4}{|c|}{d} \\
\cline{3-6}
 &   & \multicolumn{2}{|c|}{e}    & \multicolumn{2}{|c|}{f} \\
\cline{3-6}
    &   & g & h & i & j \\
\hline
  k &   &   &   &   & \\
\hline
  l &   &   &   &   & \\
\hline
\end{tabular}

\vskip 20pt

\begin{tabular}{|c|c|c|c|c|c|}
\hline
\multirow{3}*{\backslashbox{a}{b}} & \multirow{3}*{c} &\multicolumn{4}{|c|}{d} \\
\cline{3-6}
 &   & \multicolumn{2}{|c|}{e}    & \multicolumn{2}{|c|}{f} \\
\cline{3-6}
    &   & g & h & i & j \\
\hline
  k &   &   &   &   & \\
\hline
  l &   &   &   &   & \\
\hline
\end{tabular}


\end{document}
