Previous research has shown that head-mounted display users experience sickness symptoms. However, many studies have used contents with fast motion, which might mask more subtle effects caused by the head-mounted display properties, like optical design or head tracking. To investigate the symptoms caused by reasons other than fast motion in contents, we used 360-degree videos without fast motion components. In a between-subjects experiment, the participants viewed 360-degree videos for 5, 10, or 20 minutes with Samsung Gear head-mounted display. The Simulator Sickness Questionnaire (SSQ) results indicate that symptom levels started to rise between 10 and 20 minutes’ viewing time. The symptom profiles showed that disorientation symptoms dominated, followed by oculomotor and nausea symptoms. Cluster analysis revealed the presence of a high symptom group and a low symptom group in participants. In the former, the symptom levels increased with viewing time, while in the latter the symptoms remained mild. Based on the results, we can conclude that viewing time between 10 and 20 minutes is critical for the development of sickness symptoms for sensitive users.