diff --git a/optuna_dashboard/ts/components/GraphRank.tsx b/optuna_dashboard/ts/components/GraphRank.tsx
index 7164b99b..6daaf10a 100644
--- a/optuna_dashboard/ts/components/GraphRank.tsx
+++ b/optuna_dashboard/ts/components/GraphRank.tsx
@@ -152,13 +152,21 @@ const getRankPlotInfo = (
   const zValues: number[] = []
   const isFeasible: boolean[] = []
   const hovertext: string[] = []
+  const convertTrialValueToNumber = (value: TrialValueNumber): number => {
+    // TrialValueNumber takes `number`, "inf", or "-inf".
+    return typeof value === "number"
+      ? value
+      : value.includes("-")
+      ? -Infinity
+      : Infinity
+  }
   filteredTrials.forEach((trial, i) => {
     const xValue = xAxis.values[i]
     const yValue = yAxis.values[i]
     if (xValue && yValue && trial.values) {
       xValues.push(xValue)
       yValues.push(yValue)
-      const zValue = Number(trial.values[objectiveId])
+      const zValue = convertTrialValueToNumber(trial.values[objectiveId])
       zValues.push(zValue)
       const feasibility = trial.constraints.every((c) => c <= 0)
       isFeasible.push(feasibility)