An alarm system for a vehicle, comprising: when the vehicle is running at a low velocity, an alarm line setting part that sets a right alarm line and a left alarm line extending forward from the vehicle, wherein the right alarm line and the left alarm line extend in the forward direction of the vehicle from a base line as a base end, the base line being a virtual line set rearward from a front edge of the vehicle by a predetermined distance; an object detection part that detects an object on a side of the vehicle, wherein the object detection part sets two detection regions (A1, A2) at the side of the right and left alarm lines (61, 62) and detects the object in one of the two detection regions (A1, A2), and wherein the object detection part detects the object in one of the two detection regions (A1, A2) based on a signal received from a side radar, wherein the side radar transmits a measuring wave to the side of the vehicle and receives a reflection wave from the object; a collision determination part that determines whether or not the object approaching the vehicle in one of the two detection regions (A1, A2) crosses one of the two alarm lines (61, 62) within a predetermined time; an alarm unit control part that activates an alarm unit when the collision determination part determines that the object crosses one of the two alarm lines (61, 62) within the predetermined time, wherein the alarm unit control part performs an alarm reduction control to reduce operation of the alarm unit when the vehicle is decelerating compared with when the vehicle is not decelerating, wherein the alarm unit control part activates the alarm unit when a velocity of the vehicle is equal to or less than a predetermined velocity; and the alarm unit control part does not perform the alarm reduction control when the vehicle starts after having decelerated from a velocity greater than the predetermined velocity and stopped. when the vehicle is running at a low velocity, an alarm line setting part that sets a right alarm line and a left alarm line extending forward from the vehicle, wherein the right alarm line and the left alarm line extend in the forward direction of the vehicle from a base line as a base end, the base line being a virtual line set rearward from a front edge of the vehicle by a predetermined distance; an object detection part that detects an object on a side of the vehicle, wherein the object detection part sets two detection regions (A1, A2) at the side of the right and left alarm lines (61, 62) and detects the object in one of the two detection regions (A1, A2), and wherein the object detection part detects the object in one of the two detection regions (A1, A2) based on a signal received from a side radar, wherein the side radar transmits a measuring wave to the side of the vehicle and receives a reflection wave from the object; a collision determination part that determines whether or not the object approaching the vehicle in one of the two detection regions (A1, A2) crosses one of the two alarm lines (61, 62) within a predetermined time; an alarm unit control part that activates an alarm unit when the collision determination part determines that the object crosses one of the two alarm lines (61, 62) within the predetermined time, wherein the alarm unit control part performs an alarm reduction control to reduce operation of the alarm unit when the vehicle is decelerating compared with when the vehicle is not decelerating, wherein the alarm unit control part activates the alarm unit when a velocity of the vehicle is equal to or less than a predetermined velocity; and the alarm unit control part does not perform the alarm reduction control when the vehicle starts after having decelerated from a velocity greater than the predetermined velocity and stopped.