This sounds like some kind of contradiction at first glance, but in fact there is no contradiction. An examination of Scripture reveals that the phrase "Son of Man" carries broad significance.
First of all, even if the phrase "Son of Man" is a reference to Jesus' humanity, it is not a denial of His deity. By becoming a man, Jesus did not cease being God. The incarnation of Christ did not involve the subtraction of deity, but the addition of humanity. Jesus clearly claimed to be God on many occasions (Matthew 16:16, 17; John 8:58; 10:30). But in addition to being divine, He was also human (see Philippians 2:6-8). He had two natures (divine and human) conjoined in one person.
Further, Scripture indicates that Jesus was not denying His deity by referring to Himself as the Son of Man. In fact, it is highly revealing that the term "Son of Man" is used in Scripture in contexts of Christ's deity. For example, the Bible says that only God can forgive sins (Isaiah 43:25; Mark 2:7). But as the "Son of Man," Jesus had the power to forgive sins (Mark 2:10). Likewise, Christ will return to earth as the "Son of Man" in clouds of glory to reign on earth (Matthew 26:63-64). In this passage, Jesus is citing Daniel 7:13 where the Messiah is described as the "Ancient of Days," a phrase used to indicate His deity (cf. Daniel 7:9).
Further, when Jesus was asked by the high priest whether He was the "Son of God" (Matthew 26:63), He responded affirmatively, declaring that He was the "Son of Man" who would come in power and great glory (verse 64). This indicated that Jesus Himself used the phrase "Son of Man" to indicate His deity as the Son of God.
Finally, the phrase "Son of Man" also emphasizes who Jesus is in relation to His incarnation and His work of salvation. In the Old Testament (Leviticus 25:25, 26, 48, 49; Ruth 2:20), the next of kin (one related by blood) always functioned as the "kinsman-redeemer" of a family member who needed redemption from jail. Jesus became related to us "by blood" (that is, He became a man) so He could function as our Kinsman-Redeemer and rescue us from sin.
