A tripod constant-velocity joint comprising: an outer joint member having a cavity elongated in a longitudinal direction and three roller tracks; an inner joint member having a center body and three journals outwardly radially protruded from the center body; and three roller assemblies respectively coupled to the journal in a state of being respectively disposed in the roller tracks, wherein the respective roller assembly comprises: an inner race which is connected to the journal and is provided with a first inner ball groove and a second inner ball groove on both lateral sides thereof; a first ball array and a second ball array respectively having a plurality of balls (32a, 32b, 32c, 33a, 33b, 33c) which are respectively disposed in the first inner ball groove and in the second inner ball groove; and a ball cage which restricts movements of the first ball array and the second ball array in a longitudinal direction thereof; wherein the first ball array is disposed in a first ball track which is formed by the first inner ball groove and a first outer ball groove formed in a corresponding lateral side surface of the roller track and the second ball array is disposed in a second ball track which is formed by the second inner ball groove and a second outer ball groove formed in a corresponding lateral side surface of the roller track, and wherein the first ball track is configured such that a sectional area thereof becomes larger in a direction toward an open side of the outer joint member and the second ball track is configured such that a sectional area thereof becomes smaller in a direction toward the open side of the outer joint member; characterized in that diameters of the plurality of the ball (32a, 32b, 32c) composing the first ball array become larger in a direction toward the open side of the outer joint member and diameters of the plurality of the ball (33a, 33b, 33c) composing the second ball array become smaller in a direction toward the open side of the outer joint member. an outer joint member having a cavity elongated in a longitudinal direction and three roller tracks; an inner joint member having a center body and three journals outwardly radially protruded from the center body; and three roller assemblies respectively coupled to the journal in a state of being respectively disposed in the roller tracks, wherein the respective roller assembly comprises: an inner race which is connected to the journal and is provided with a first inner ball groove and a second inner ball groove on both lateral sides thereof; a first ball array and a second ball array respectively having a plurality of balls (32a, 32b, 32c, 33a, 33b, 33c) which are respectively disposed in the first inner ball groove and in the second inner ball groove; and a ball cage which restricts movements of the first ball array and the second ball array in a longitudinal direction thereof; wherein the first ball array is disposed in a first ball track which is formed by the first inner ball groove and a first outer ball groove formed in a corresponding lateral side surface of the roller track and the second ball array is disposed in a second ball track which is formed by the second inner ball groove and a second outer ball groove formed in a corresponding lateral side surface of the roller track, and wherein the first ball track is configured such that a sectional area thereof becomes larger in a direction toward an open side of the outer joint member and the second ball track is configured such that a sectional area thereof becomes smaller in a direction toward the open side of the outer joint member; characterized in that diameters of the plurality of the ball (32a, 32b, 32c) composing the first ball array become larger in a direction toward the open side of the outer joint member and diameters of the plurality of the ball (33a, 33b, 33c) composing the second ball array become smaller in a direction toward the open side of the outer joint member. an inner race which is connected to the journal and is provided with a first inner ball groove and a second inner ball groove on both lateral sides thereof; a first ball array and a second ball array respectively having a plurality of balls (32a, 32b, 32c, 33a, 33b, 33c) which are respectively disposed in the first inner ball groove and in the second inner ball groove; and a ball cage which restricts movements of the first ball array and the second ball array in a longitudinal direction thereof; wherein the first ball array is disposed in a first ball track which is formed by the first inner ball groove and a first outer ball groove formed in a corresponding lateral side surface of the roller track and the second ball array is disposed in a second ball track which is formed by the second inner ball groove and a second outer ball groove formed in a corresponding lateral side surface of the roller track, and wherein the first ball track is configured such that a sectional area thereof becomes larger in a direction toward an open side of the outer joint member and the second ball track is configured such that a sectional area thereof becomes smaller in a direction toward the open side of the outer joint member; characterized in that diameters of the plurality of the ball (32a, 32b, 32c) composing the first ball array become larger in a direction toward the open side of the outer joint member and diameters of the plurality of the ball (33a, 33b, 33c) composing the second ball array become smaller in a direction toward the open side of the outer joint member.