A clutch apparatus, comprising: a clutch outer connected to an input power member; a clutch inner having a first ring plate portion extending along a plane perpendicular to an axial line of an output power shaft and a plurality of boss portions extending in an axial direction from the first ring plate portion and connected in an interlocking relationship to the output power shaft; a plurality of driving friction plates configured to be engaged by the clutch outer; a plurality of driven friction plates disposed alternately with the driving friction plates and configured to be engaged by the clutch inner; a pressure receiving plate disposed facing the one of the driving friction plates and the driven friction plates which is disposed at one axial end of the clutch inner and coupled to the clutch inner; a pressing plate cooperating with the pressure receiving plate to sandwich the driving friction plates and the driven friction plates therebetween; a movable cam member spline-coupled to the clutch inner so that it can move axially but cannot rotate relative thereto; a fixed cam member fixed to the output power shaft so as to cooperate with the movable cam member to form a cam mechanism and having a second ring plate portion which faces the first ring plate portion and has a plurality of openings which are provided therein so as to allow the boss portions to pass through the second ring plate portion; and a plurality of clutch springs provided between a spring receiving plate fixed to an end portion of the boss portions and the pressing plate and configured to bias the pressing plate so as to cooperate with the pressure receiving plate to press the driving friction plates and the driven friction plates together; a bulbous portion being provided integrally at a base portion of each of the boss portions in such a manner as to extend at opposite sides from the boss portion along a circumferential direction of the clutch inner such that, in a state in which a phase between the fixed cam member and the movable cam member is displaced from a correct phase, the bulbous portion abuts with an end edge of the associated opening along a circumferential direction of the fixed cam member to thereby prevent insertion of the associated boss portion into the opening. a clutch outer connected to an input power member; a clutch inner having a first ring plate portion extending along a plane perpendicular to an axial line of an output power shaft and a plurality of boss portions extending in an axial direction from the first ring plate portion and connected in an interlocking relationship to the output power shaft; a plurality of driving friction plates configured to be engaged by the clutch outer; a plurality of driven friction plates disposed alternately with the driving friction plates and configured to be engaged by the clutch inner; a pressure receiving plate disposed facing the one of the driving friction plates and the driven friction plates which is disposed at one axial end of the clutch inner and coupled to the clutch inner; a pressing plate cooperating with the pressure receiving plate to sandwich the driving friction plates and the driven friction plates therebetween; a movable cam member spline-coupled to the clutch inner so that it can move axially but cannot rotate relative thereto; a fixed cam member fixed to the output power shaft so as to cooperate with the movable cam member to form a cam mechanism and having a second ring plate portion which faces the first ring plate portion and has a plurality of openings which are provided therein so as to allow the boss portions to pass through the second ring plate portion; and a plurality of clutch springs provided between a spring receiving plate fixed to an end portion of the boss portions and the pressing plate and configured to bias the pressing plate so as to cooperate with the pressure receiving plate to press the driving friction plates and the driven friction plates together;