This paper describes SingDistVis, an information visualization technique for fundamental frequency (F0) trajectories of large-scale singing data where numerous singers sing the same song. SingDistVis allows to explore F0 trajectories interactively by combining two views: OverallView and DetailedView. OverallView visualizes a distribution of the F0 trajectories of the song in a time-frequency heatmap. When a user specifies an interesting part, DetailedView zooms in on the specified part and visualizes singing assessment (rating) results. Here, it displays high-rated singings in red and low-rated singings in blue. When the user clicks on a particular singing, the audio source is played and its F0 trajectory through the song is displayed in OverallView. We selected heatmap-based visualization for OverallView to provide an overview of a large-scale F0 dataset, and polyline-based visualization for DetailedView to provide a more precise representation of a small number of particular F0 trajectories. This paper introduces a subjective experiment using 1,000 singing voices to determine suitable visualization parameters. Then, this paper presents user evaluations where we asked participants to compare visualization results of four types of Overview+Detail designs and concluded that the presented design archived better evaluations than other designs in all the seven questions. Finally, this paper describes a user experiment in which eight participants compare SingDistVis with a baseline implementation in exploring interested singing voices and concludes that the proposed SingDistVis archived better evaluations in nine of the questions.