While the neuronal activation in the medial frontal cortex is thought to reflect higher-order evaluation processes of reward prediction errors when a reward deviates from our expectation, there is increasing evidence that the medial frontal activity might express prediction errors in general. However, given that several studies examined the medial frontal event-related potentials (ERPs) by comparing signals triggered by different stimuli and different anticipations, it remains an open question whether the medial frontal signals are sensitive to the valence of prediction errors. Here we orthogonally manipulated expectation magnitude (i.e., large/small expectation) and expectation confirmation (i.e., fulfilled/violated expectation) in a target detection task with rewards. We found that the medial frontal ERPs were more negative-going for unexpected outcomes in comparison with expected outcomes, regardless of whether a large/small reward was expected. The result supports the idea that the medial frontal signals express prediction errors in general regardless of their valence.