The differential effects of rumination on attentional breadth was investigated by experimentally manipulating rumination from the perspective of thought content and valences of thinking-time. We randomly assigned 68 undergraduate and graduate students to negative, neutral, long-, and short-time thinking groups. Then, we administered a questionnaire, conducted a rumination manipulation and the modified Attentional Breadth Task. In this task, there are Close and Far conditions that have a narrow and wide attentional range depending on the location of the target stimulus, and ΔAttentional Narrowing Index (ΔANI) representing the differences between the correct response rate for a target in Close and Far conditions are calculated, such that higher ΔANI values indicate a narrower attention range. Results indicated that state rumination and negative emotions worsened and attentional breadth decreased when participants focused on negative thoughts for a long time. Moreover, a causal relationship between rumination and attentional breadth was suggested. Also, factors narrowing attentional breadth through rumination were identified based on the attentional scope model. It is suggested that future studies should consider whether depression or negative cognitive processing is worsened by narrowing attentional breadth.