A stack unit for a signal indicator lamp including a plurality of such stack units stacked in a predetermined stacking direction, the stack unit comprising: an outer member having a first internal space and a pair of first openings through which the first internal space is open to opposite sides defined with respect to the stacking direction; a connection portion provided on the outer member for connecting the outer member to another stack unit of the signal indicator lamp by rotating the outer member about a rotation axis extending in the stacking direction; an inner member provided in the first internal space and having a second internal space and a pair of second openings through which the second internal space is open to the opposite sides defined with respect to the stacking direction; a support portion provided on the outer member for supporting the inner member so that the inner member is rotatable relative to the outer member about the rotation axis and immovable relative to the outer member in the stacking direction; a board having a pair of parallel major surfaces and held in the second internal space by the inner member with the major surfaces extending in the stacking direction; a first connector provided at an end of the board on one of the opposite sides defined with respect to the stacking direction for external electrical connection; a second connector provided at an end of the board on the other side defined with respect to the stacking direction for external electrical connection; a restriction portion having a one-side engagement portion provided in an end portion of the inner member on the one side and an other-side engagement portion provided in an end portion of the inner member on the other side for restricting relative rotation of the inner members of adjacent stack units of the signal indicator lamp about the rotation axis with the other-side engagement portion of one of the adjacent stack units in engagement with the one-side engagement portion of the other stack unit, wherein the one-side engagement portion includes a plurality of projections each extending in the stacking direction, wherein the other-side engagement portion includes a plurality of trenches each extending in the stacking direction for respectively receiving the projections of the one-side engagement portion of the other stack unit, and wherein the trenches have different trench widths. an outer member having a first internal space and a pair of first openings through which the first internal space is open to opposite sides defined with respect to the stacking direction; a connection portion provided on the outer member for connecting the outer member to another stack unit of the signal indicator lamp by rotating the outer member about a rotation axis extending in the stacking direction; an inner member provided in the first internal space and having a second internal space and a pair of second openings through which the second internal space is open to the opposite sides defined with respect to the stacking direction; a support portion provided on the outer member for supporting the inner member so that the inner member is rotatable relative to the outer member about the rotation axis and immovable relative to the outer member in the stacking direction; a board having a pair of parallel major surfaces and held in the second internal space by the inner member with the major surfaces extending in the stacking direction; a first connector provided at an end of the board on one of the opposite sides defined with respect to the stacking direction for external electrical connection; a second connector provided at an end of the board on the other side defined with respect to the stacking direction for external electrical connection; a restriction portion having a one-side engagement portion provided in an end portion of the inner member on the one side and an other-side engagement portion provided in an end portion of the inner member on the other side for restricting relative rotation of the inner members of adjacent stack units of the signal indicator lamp about the rotation axis with the other-side engagement portion of one of the adjacent stack units in engagement with the one-side engagement portion of the other stack unit, wherein the one-side engagement portion includes a plurality of projections each extending in the stacking direction, wherein the other-side engagement portion includes a plurality of trenches each extending in the stacking direction for respectively receiving the projections of the one-side engagement portion of the other stack unit, and wherein the trenches have different trench widths.