An electric linear motion actuator comprising: an electric motor having a motor case; a drive shaft configured to be rotated by the electric motor; a rotation-to-linear motion converting mechanism configured to convert a rotational motion of the drive shaft to an axial linear motion of an axially movably supported sliding member; and an electric locking mechanism incorporated in a power transmission path through which power is transmitted from the electric motor to the drive shaft, wherein the locking mechanism is configured, when energized, to shift the power transmission path to a locked state in which power cannot be transmitted through the power transmission path, and configured, when de-energized thereafter, to keep the power transmission path in the locked state, wherein the electric locking mechanism comprises: a locking pin movable toward and away from one point on a rotation trajectory circle of an engagement portion formed in a side surface of a rotary member for transmitting power; and a linear solenoid configured to move the locking pin toward and away from said one point, characterized in that the electric locking mechanism is built in the motor case of the electric motor, and an anti-rotation means is provided between the linear solenoid and the motor case, wherein the anti-rotation means comprises: an axial engagement groove formed in an inner periphery of a solenoid receiving hole formed in the motor case; and an anti-rotation protrusion formed on an outer periphery of the linear solenoid and engaged in the engagement groove. an electric motor having a motor case; a drive shaft configured to be rotated by the electric motor; a rotation-to-linear motion converting mechanism configured to convert a rotational motion of the drive shaft to an axial linear motion of an axially movably supported sliding member; and an electric locking mechanism incorporated in a power transmission path through which power is transmitted from the electric motor to the drive shaft, wherein the locking mechanism is configured, when energized, to shift the power transmission path to a locked state in which power cannot be transmitted through the power transmission path, and configured, when de-energized thereafter, to keep the power transmission path in the locked state, wherein the electric locking mechanism comprises: a locking pin movable toward and away from one point on a rotation trajectory circle of an engagement portion formed in a side surface of a rotary member for transmitting power; and a linear solenoid configured to move the locking pin toward and away from said one point, characterized in that the electric locking mechanism is built in the motor case of the electric motor, and an anti-rotation means is provided between the linear solenoid and the motor case, a locking pin movable toward and away from one point on a rotation trajectory circle of an engagement portion formed in a side surface of a rotary member for transmitting power; and a linear solenoid configured to move the locking pin toward and away from said one point, characterized in that the electric locking mechanism is built in the motor case of the electric motor, and an anti-rotation means is provided between the linear solenoid and the motor case, wherein the anti-rotation means comprises: an axial engagement groove formed in an inner periphery of a solenoid receiving hole formed in the motor case; and an anti-rotation protrusion formed on an outer periphery of the linear solenoid and engaged in the engagement groove. an axial engagement groove formed in an inner periphery of a solenoid receiving hole formed in the motor case; and an anti-rotation protrusion formed on an outer periphery of the linear solenoid and engaged in the engagement groove.