We analyze the pattern of inter-regional functional association between cortical activities during auditory oddball tasks, and the influence of task difficulty on it. Event-related electroencephalograms were recorded from 17 subjects during auditory oddball tasks with two task difficulty levels. The task difficulty was controlled by changing the difference between the frequencies of standard and target tones. The changes in behavioral response and P300 component due to the difficulty were consistent with previous findings, whereby successful control of difficulty was verified. Significant gamma- and theta-band phase synchronization (PS) was observed primarily between frontal and posterior electrodes along the midline, which is interpreted as functional connectivity among cortical regions devoted to the task execution. Apparent differences in PS were identified between two difficulties in both gamma- and theta-bands. On the whole, the number of electrode pairs showing significant PS was much smaller for higher task difficulty. The overall result is in agreement with our recent study which reported similar difference in PS due to the difficulty of 'visual' oddball task.