An internal gear grinding machine for use in grinding a to-be-machined internal gear, comprising: a barrel-shaped threaded tool having a diameter gradually decreasing from an axially intermediate portion of the threaded tool toward axially opposite end portions thereof, wherein the internal gear grinding machine is configured to grind the to-be-machined internal gear by synchronously rotating the to-be-machined internal gear and the barrel-shaped threaded tool in mesh with each other, wherein the to-be-machined internal gear is rotatable about a work rotation axis, wherein the barrel-shaped threaded tool is rotatable about a tool rotation axis having a predetermined crossed axes angle (Σ) with respect to the work rotation axis; tool turning means for turning the threaded tool about a tool pivot axis perpendicular to the work rotation axis; dressing means for dressing the threaded tool, the dressing means comprising: tool rotating means for rotating the threaded tool about the tool rotation axis; tool vertically-moving means for moving the threaded tool in a vertical direction; and tool horizontally-moving means for moving the threaded tool to adjust a distance between the tool rotation axis and the dresser rotation axis; characterized in that the dressing means is configured to dress the threaded tool by meshing the threaded tool with a disk-shaped dresser rotatable about a dresser rotation axis, and the dressing means comprises dresser turning means for turning the dresser about a horizontal dresser pivot axis perpendicular to the dresser rotation axis; and tool laterally-moving means for moving the threaded tool in a direction perpendicular to both the tool rotation axis and a direction in which to adjust the distance between the tool rotation axis and the dresser rotation axis, wherein the internal gear grinding machine is configured to drive the dresser turning means, the tool rotating means, the tool vertically-moving means, the tool horizontally-moving means, and the tool laterally-moving means in accordance with the shape of the threaded tool. a barrel-shaped threaded tool having a diameter gradually decreasing from an axially intermediate portion of the threaded tool toward axially opposite end portions thereof, wherein the internal gear grinding machine is configured to grind the to-be-machined internal gear by synchronously rotating the to-be-machined internal gear and the barrel-shaped threaded tool in mesh with each other, wherein the to-be-machined internal gear is rotatable about a work rotation axis, wherein the barrel-shaped threaded tool is rotatable about a tool rotation axis having a predetermined crossed axes angle (Σ) with respect to the work rotation axis; tool turning means for turning the threaded tool about a tool pivot axis perpendicular to the work rotation axis; dressing means for dressing the threaded tool, the dressing means comprising: tool rotating means for rotating the threaded tool about the tool rotation axis; tool vertically-moving means for moving the threaded tool in a vertical direction; and tool horizontally-moving means for moving the threaded tool to adjust a distance between the tool rotation axis and the dresser rotation axis; tool rotating means for rotating the threaded tool about the tool rotation axis; tool vertically-moving means for moving the threaded tool in a vertical direction; and tool horizontally-moving means for moving the threaded tool to adjust a distance between the tool rotation axis and the dresser rotation axis; characterized in that the dressing means is configured to dress the threaded tool by meshing the threaded tool with a disk-shaped dresser rotatable about a dresser rotation axis, and the dressing means comprises dresser turning means for turning the dresser about a horizontal dresser pivot axis perpendicular to the dresser rotation axis; and tool laterally-moving means for moving the threaded tool in a direction perpendicular to both the tool rotation axis and a direction in which to adjust the distance between the tool rotation axis and the dresser rotation axis, dresser turning means for turning the dresser about a horizontal dresser pivot axis perpendicular to the dresser rotation axis; and tool laterally-moving means for moving the threaded tool in a direction perpendicular to both the tool rotation axis and a direction in which to adjust the distance between the tool rotation axis and the dresser rotation axis, wherein the internal gear grinding machine is configured to drive the dresser turning means, the tool rotating means, the tool vertically-moving means, the tool horizontally-moving means, and the tool laterally-moving means in accordance with the shape of the threaded tool.