A physical mechanism that drives FU Orionis-type outbursts is reconsidered .
We study the effect of inner part of a circumstellar disk covering a region from near the central star to the radius of approximately 5 AU ( hereafter , the inner disk ) .
Using the fluctuated mass accretion rate onto the inner disk \dot { M } _ { out } , we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star \dot { M } _ { in } by means of numerical calculation of an unsteady viscous accretion disk in a one-dimensional axisymmetric model .
First , we calculate the evolution of the inner disk assuming an oscillating \dot { M } _ { out } .
It is shown that the time variability of \dot { M } _ { in } does not coincide with \dot { M } _ { out } due to viscous diffusion .
Second , we investigate the properties of spontaneous outbursts with temporally constant \dot { M } _ { out } .
Outburst occur only in a limited range of mass accretion rates onto the inner disk 10 ^ { -10 } < \dot { M } _ { out } < 3 \times 10 ^ { -6 } ~ { } { M } _ { \odot } { yr } ^ { -1 } due to gravo-magneto limit cycle ( GML ) .
Finally , we discuss the case with a combination of episodic \dot { M } _ { out } and accretion outbursts cause by the GML in the inner disk .
The GML can drive accretion outbursts onto the star even for the case of fluctuating \dot { M } _ { out } , although fluctuations of \dot { M } decay during transmitting the inner disk inwards .
We newly identified two modes of outburst which are spontaneous one and stimulated one .
In a stimulated mode of outburst , \dot { M } _ { out } does appear directly in \dot { M } _ { in } ( the latter defining the stellar accretion luminosity ) .
In a spontaneous mode of outburst , \dot { M } _ { out } appears as the interval between outbursts .