Can MSSM Particle be the Inflaton?

We consider the possibility of using one of the $D$-flat directions in the minimal supersymmetric standard model (MSSM) as the inflaton. We show that the flat direction consisting of (first generation) left- and right-handed up-squarks as well as the up-type Higgs boson may play the role of the inflaton if dominant part of the up-quark mass is radiatively generated from supersymmetric loop diagrams. We also point out that, if the R-parity violating Yukawa coupling is of $O(10^{-7})$, $R$-odd $D$-flat directions may be another possible candidate of the inflaton. Such inflation models using $D$-flat directions in the MSSM are not only testable with collider experiments but also advantageous to resolve the problem how the inflaton reheats the universe.