diff --git a/itf/src/value/bigint.rs b/itf/src/value/bigint.rs
index 9ca483b..7cfc68b 100644
--- a/itf/src/value/bigint.rs
+++ b/itf/src/value/bigint.rs
@@ -55,13 +55,31 @@ impl<'de> Deserialize<'de> for BigInt {
         D: serde::Deserializer<'de>,
     {
         #[derive(Deserialize)]
-        struct BigInt {
-            #[serde(rename = "#bigint")]
-            bigint: String,
+        #[serde(untagged)]
+        enum BigInt {
+            // deserialized serde_json::Value
+            Itf {
+                #[serde(rename = "#bigint")]
+                bigint: String,
+            },
+            // deserialized itf::Value
+            BigInt(i64, Vec<u32>),
         }
 
-        let inner = BigInt::deserialize(deserializer)?;
-        let bigint = inner.bigint.parse().map_err(serde::de::Error::custom)?;
-        Ok(Self(bigint))
+        match BigInt::deserialize(deserializer)? {
+            BigInt::Itf { bigint } => {
+                let bigint: num_bigint::BigInt =
+                    bigint.parse().map_err(serde::de::Error::custom)?;
+                Ok(Self::new(bigint))
+            }
+            BigInt::BigInt(sign, digits) => {
+                let sign = match sign.cmp(&0) {
+                    std::cmp::Ordering::Less => num_bigint::Sign::Minus,
+                    std::cmp::Ordering::Equal => num_bigint::Sign::NoSign,
+                    std::cmp::Ordering::Greater => num_bigint::Sign::Plus,
+                };
+                Ok(Self::new(num_bigint::BigInt::new(sign, digits)))
+            }
+        }
     }
 }