This study examined how working memory plays different roles in open-ended versus closed-ended creative problem-solving processes, as represented by divergent thinking tests and insight problem-solving tasks. With respect to the analysis of different task demands and the framework of dual-process theories, the hypothesis was that the idea generation in a divergent thinking test relies more on associative, effortless system 1 processing, but insight problem solving requires rule-based, resource-limited system 2 processing, in addition to system 1 processing. Since system 1 was suggested to be more active in resource deprivation conditions, Experiment 1 adopted the dual-task paradigm, which increased participants' working memory load. The results showed that divergent thinking performance was enhanced and insight problem-solving performance was hindered. Experiment 2 using the individual differences approach found that individuals' working memory capacity correlated with insight problem solving but not with divergent thinking performance, indicating a possible involvement of system 2 processing in insight problem solving. These findings suggested that open-ended and closed-ended creative problem solving involve different processes and helped to clarify some past inconsistencies when considering the relationship of factors with creativity.