  The Theta+ was searched for via the K+p -> pi+X reaction using the 1.2 GeV/c
K+ beam at the K6 beam line of the KEK-PS 12 GeV Proton Synchrotron. In the
missing mass spectrum of the K+p -> pi+X reaction, no clear peak structure was
observed. Therefore a 90 % C.L. upper limit of 3.5 ub/sr was derived for the
differential cross section averaged over 2degree to 22degree in the laboratory
frame of the K+p -> pi+Theta+ reaction. This upper limit is much smaller than
the theoretical calculation for the t-channel process where a K0* is exchanged.
  From the present result, either the t-channel process is excluded or the
coupling constant of g_{K*N\Theta} is quite small.
