A lamp including a first case, a second case, a lamp bottom and a pair of friction units is provided. The second case has a first sidewall and a second sidewall perpendicularly to the first sidewall. The first sidewall has a first positioning portion extending along a normal direction to the first sidewall, and the second case is detachably assembled with the first case to form a case assembly having a first opening. The lamp bottom is embedded into the first opening. The lamp bottom has a second opening and a second positioning portion, wherein the second positioning portion is located on the path of the first positioning portion when the case assembly rotates relative to the lamp bottom. The friction units are disposed at opposing sides of the first opening and contact a portion of the lamp bottom embedded into the first opening.