We consider all or nothing investment problem with a nite time horizon when the investment opportunity set is changing stochastically over time, especially under Markovian regime-switching environment, and a decision maker faces ambiguity of parameters governing prot ow dynamics of the investment. We apply -Maxmin Expected Utility(-MEU) preferences to reect the ambiguity seeking attitude of de-cision maker and provide semi-explicit formulas for the expected value of investment and the critical present value of the prot ow. Numerical results show that the crit-ical present value of the protow depends on the business cycle and the tendency of ambiguity seeking is mitigated in case of project whose prot ow is dependent on regime-switching environment.