Provided are: a linear actuator which is configured so that, when rectilinearly moving an output member utilizing the differential motion between two gears, a drive source and the output member can be mechanistically connected to and disconnected from each other using a simple configuration; and a lid lock device. A linear actuator (1) is configured such that a motor (20) rotates and drives a drive gear (30) (first drive gear (31) and second drive gear (32)). A first driven gear (41) meshes with the first drive gear (31), and a second driven gear (42) meshes with the second drive gear (32). The gear ratio between the first drive gear (31) and the first driven gear (41) and the gear ratio between the second drive gear (32) and the second driven gear (42) are different, and the second driven gear (42) is rectilinearly moved in the axial direction (L) utilizing the differential motion between the first driven gear (41) and the second driven gear (42). A clutch member moves the first drive gear (31) and the second drive gear (32) in the axial direction (L) to release the mesh between the first drive gear (31) and first driven gear (41).